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

Or do a double blind listening test?
But as others mentioned this may or may not be worth spending time on.
It's totally up to you.

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.

...and besides, the bottom line is the listeners don't care. As I often say, no radio station ever refused to play one of my records because it used a solid-state mic preamp.

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

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

Yes, the changes since it went 64-bit have been about efficiency.

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. 

Something to keep in mind - the audio engine isn't really doing anything that one should expect to hear differences between versions:
 
1. Summing tracks is literally summing - you just add the samples from each track together and that's all it is.
 
2. Level changes (including pan) is just multiplication - lowering gain means you multiply each sample by a number less than one and increasing gain means you multiply each sample by a number greater than one and that's all it is.
 
 
With 32 bit precision the calculations are effectively perfect from a human perspective (meaning calculation errors are almost certainly buried below other noise and also below the threshold of hearing). And with 64 bit double precision enabled the calculation errors are effectively mathematically perfect to far beyond 24 bits (meaning any calculation errors won't ever make it into the output).
 
Anything other than this would mean the audio engine wasn't designed to be clean and was instead trying to deliberately color the sound. Unless there's a bug, Sonar doesn't do that - unless you use plugins/EQ/console emulator/etc. to tell it to.

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

Great stuff, I like Subtlearts explanation(s).
 

chuckebaby
I could be wrong but I believe the Capstan rollers in Sonar 5 were higher quality than Sonars newer versions.
thus less wow and flutter .

Chuck, I got the swab & alcohol, but I can't find the capstan or the heads or anything.
And I checked the Tape Emulator.