SonicExplorer
If listening isn't enough what's the right way to be certain then? Should I take the two rendered files and re-import them into a project, phase invert one of the tracks, and then bounce out again and inspect the result visually to make sure it is dead flat-line? 

this is what i meant, yes
 
but as the others say, if it sounds ok then it is ok

Having been in audio clubs in which we sound-test cool speakers and amps and, yes, cables and such, I've seen enough to say, with confidence, that the very first thing one should do is a blind test.  Have someone queue up the music without telling you which one, then you say which.  Have them flip a coin on their own to choose which one to play.  Do this twenty times for at least a modicum of statistical breadth, then check the results.
 
My most famous run with this kind of thing earned me $100.  A friend said he could DEFINITELY tell the difference between a famous Canadian amplifier and a mid-priced Japanese amp.  He bet $100.  We did the test and not only did he completely not know which amp was which, he would vehemently shout "That's the Canadian League there, boys!" when the other amp was blasting the music.
 
This is not to say there aren't differences in your files/exports/Sonars; it's just to say that the very simplest thing to start with is a blind test.  If that is miserable for you, then the rest is explained.

is not whatever Sonar version have the same (audio engine) ??

I suspect implicit to this discussion was a question of whether or not the audio engine was improved in S6 over S5.  Listening on playback my ears say yes, but that's not what the rendered files seem to indicate thus far.   However, I still need to do a bit more testing by re-importing the rendered files and inspecting more closely. 

Guys, I spent some time today and determined the kick & snare hits are indeed very audible on the phase inversion test, so I need to look into that further.  BUT, as for guitar and bass, still nothing audible.  But, when I bounced out a section of the rendered tracks with only bass and guitar playing, and then re-imported the result, the graphic does not visually look like a perfect flatline as I expected.  I was hoping some of you guys might take a look/listen to a short .WAV file, but I don't know how to attach it here. What am I missing? 
 
Sonic

LoL   Ok, I like a joke as much as anybody but the latest findings indicate I was indeed hearing a difference after all.  I had chalked the existence of drum hits up to the fact LiveSynthPro doesn't work reliably with multi-core CPU's, but it must be more than because I am only using LSP on the kick and yet I'm hearing both kick AND snare hits in the phase reverse test.  While DFH Superior is the primary drum source I also use Sound Fonts for kicks sometimes.  Anyway, beyond that however, the original findings remain in place WRT the bass & guitar tracks - there is no readily "audible" difference but yet there appears to MAYBE be a difference in reality because the tracks are not perfectly flatlined.  So I'd really like to have you guys take a look & listen, if somebody would explain how to attach a short .WAV file to this thread.... ??
 
Sonic

If the audio is generated from a soft synth, then that may also be a reason there is a difference. Especially with drums, as they sometimes 'humanize' things. Meaning, small differences in performance every time, like a real drummer. If these are rendered tracks, imported into each version, then this does not apply. But, when generated in each sonar version, there might be a difference that causes the tracks not to 'flatline'. In superior drummer, there are some settings to tweak what it actually does, but changes in velocity and tempo are definitely on by default. 
Also, some effects (especially the ones that emulate vintage equipment) add some random element as well. 
 
While these will cause a null test to fail, it would not mean a lot of difference in sound...
 
 

you can take possible dpe "efficiency changes" out of the equation by disabling it in both and trying

stxx
"so issue is in your head" - "No other explanation needed"

A little harsh, no ?
Imagine if everyone here used that as an answer ?

Thanks again guys for all the constructive input, this forum is just incredible.  
 
Nice catch on the drums...I have the humanize settings enabled in DFH Superior, so I suspect that is the culprit on the drum side.  I can investigate further, but for now, more importantly is the remaining tracks (such as guitar and bass).  I have been recording for longer than many people on the forum have probably been alive and struggled making the conversion from the big-studio analog days to the home DAW.  While I'm getting old and my hearing isn't 100% anymore, I am still able to detect very minor sound differences.  And something seems audibly different to my ears.  Especially given how sensitive I've always been to guitar distortion textures.  The digital audio world has struggled in that realm IMO so I'm always listening for how "real" the guitar tones are being captured & replicated.  And to my ears S6 is seems to be generating a slightly "softer" more realistic sounding tube amp distortion on playback.  Actually, as I said initially, the overall playback for all tracks in S6 seem to exhibit that same vibe.
 
I too really wish someone from Cakewalk would comment on this subject.  I realize S5 and S6 are older but maybe there is something interesting to be learned. Or maybe there is indeed some sort of difference on playback, if not also in the rendering.  Maybe inaudible differences using a NULL test can, in some instances, be audible in rendered tracks?  I'm simply not enough of an expert in this kind of digital minutiae.
 
Sonic

SonicExplorer
Maybe inaudible differences using a NULL test can, in some instances, be audible in rendered tracks?  I'm simply not enough of an expert in this kind of digital minutiae.
 

If something nulls perfectly it means it's exactly the same.
 
If it nulls to the point of being below the threshold of hearing it is inaudible by definition. If it nulls to the point of being sufficiently buried below the noise floor of the track, or the listening environment, or anything else it is inaudible.
 

For whatever it's worth -  I recently jumped from Sonar5 to Sonar Pro and I hear no audible difference that I can attribute to Sonar. I did check to make sure that the pan laws were the same. There are differences between DAWS though, Samplitude sounds great and does sound different, as does Mixbus. I like the sound from all of them but working in Sonar integrating midi to audio is far more advanced in my opinion.

Ok, so pan laws and dithering are potential possibilities.  Please add anything else to the list you guys can think of that are worth checking.   I'll wait a day or so and then take any suggestions and look into things....
 
Sonic

SonicExplorer
...is the only viable explanation that it is all in my mind?

 
It was in my mind too. I originally started with Sonar 6 and I do remember that there were differences between playback and export in that version. Also, playback of 24-bit mixes sounded hugely dynamic against 16-bit versions of the same mixes, which sounded flat and lifeless in comparison. After upgrading to Sonar 7 and beyond, these differences were no longer apparent.

pan laws also got some attention at some point