I don't know what you all are seeing but mono-to-stereo absolutely does NOT work correctly with SONAR.  The plug-in will process as stereo but then the right channel gets thrown out and SONAR plays back the left as mono.  This is easy to confirm with SuperTap by dragging the direct signal back and forth and noticing that it goes silent as it is panned right.  If you are having difficulty getting what you want out of the plug-in, this is likely why.  Certain combos of plug-ins might change this behavior slightly -- I think I may have seen chains that resulted in mono-to-stereo summing the output to mono -- but I haven't found a combo that ever produces correct results.  The Stereo versions of the plug-ins with stereo interleave enabled will get you what you want.
 
Regarding the original post, I've had occasional issues where it seems like SONAR and the plug-in get out of sync with the stereo interleave settings.  I've always found that disabling and re-enabling the plug-in gets it working correctly (using the SONAR on/off button -- don't know if host bypass does the same thing).  
 

Marshall

Keni
Mono-stereo works in sonar

I've been playing with this plugin a bit the last few days and find it difficult to get what I want...

I'm sure that's just me, but just saying...

Groove 3 have some excellent videos on its use. 

F.w.i.w....The SuperTap videos are part of the Groove 3 Waves Musicians Bundle tutorial.
 
Rocky

There's really no need to use any mono versions at all of the Waves stuff. They are included mainly for legacy support but Sonar will automatically feed a mono signal into a stereo plugin correctly. So use the stereo version of SuperTap not the mono-to-stereo.

This is worse than I thought - I can only get a mono output from the full stereo versions of both Waves Doubler and SuperTap when I use it as an insert. If I drag the plugin to a bus and send the signal there it works fine. This is not good, as I want totally different settings for voice, guitar etc and don't want to set up loads of unnecessary buses! All interleaves set to stereo. 

Marshall, Just gave this a test. I think you are correct. Supertap 6 (what you are using) is not panning correctly.
However, Supertap 2 stereo is working as it should. you can hear a big difference in the stereo spectrum by doing examples with either one. Tap 6 seems dead in the sides and only delaying in the middle.
Tap 2 Stereo is very broad an wide. ping pong is all there. Don't need a Groove 3 videos to figure this out.
Anyway. I hope this helps.

Marshall
This is worse than I thought - I can only get a mono output from the full stereo versions of both Waves Doubler and SuperTap when I use it as an insert. If I drag the plugin to a bus and send the signal there it works fine. This is not good, as I want totally different settings for voice, guitar etc and don't want to set up loads of unnecessary buses! All interleaves set to stereo. 

You should let both Sonar and Waves official supports know about this by submitting tickets.

For me, neither SuperTap 2 or SuperTap 6 are working correctly, unless they are set up as sends. If anyone else wishes to verify their experience please do, and I will raise with Waves and Cakewalk support. 

I partially replicated your problem as described.
 
Mono/Stereo will only send out one signal, while Stereo seems to send both.
 
Is your guitar track mono? If so, interleave it to Stereo and the Stereo insert should work.

I can also confirm that none of the SuperTap variants output stereo when inserted on a mono track with a mono source, regardless of track interleave.
 
I'd suggest substituting the Sonitus Delay, assuming you just want a basic ping-pong delay without separate L/R EQ or panning, the only advantages SuperTap offers over the Sonitus.

Huh weird I never noticed. I mostly use H-Delay though.