If I have a piece in 6/8 time in Sonar 8.5, the view of the Piano Roll seems to divide the bar into 3 equal parts. But my understanding of 6/8, and the usual interpretation it seems, is that 6/8 is 2 groups of 3 eighth notes - not 3 groups of 2 eighth notes. Thus, the view in the Piano Roll is a bit misleading because it has these apparent accents on the 3rd and 5th beats which don't exist in practice. Is there any way to change this?

Not that I've been able to find.
 
If you set the Snap to "Dotted Quarter" and set the Show Grid to "Follow snap settings" you'll get a single line dividing the measure in the right spot (on beat 4) but then of course any notes you enter snap to the lines on beat 1 or 4.
 
So far the best compromise I can find is to set the Show Grid to eighths and then you see six lines per measure instead of just 3 although the red lines will still be at beats 3 and 5. I generally set my cursor at the beginning of the measure I'm working on as a visual reference.
 
It's unfortunate the Show Grid settings aren't nearly as varied as the Snap To Grid settings. Considering how common x/8 music is you should be able to set the "accent" line somewhere besides every second eighth note. As you say, minor issue, but it would be nice.
 
Hopefully someone will chime in with something I've missed.

But my understanding of 6/8, and the usual interpretation it seems, is that 6/8 is 2 groups of 3 eighth notes - not 3 groups of 2 eighth notes.

Normally, yes, but it could be 3 groups of 2 or a group of 4 followed by a group of 2 etc. 5/4 for example could be subdivided into 3+2 or 2+3.


I'm not a big PRV user, but I believe to accomplish what you want, set the Grid Resolution in PRV to 1/8 triplet. You should also set the Snap To Grid to 1/8 note triplet. 

6/8 could certainly be counted differently, but since the usual usage is 2 sets of 3, I wish Sonar defaulted to that presentation, that's all. When the grid and snap resolution is set to 1/8 triplet in 6/8, it divides the bar into 9 spaces, 3 sets of 3. I have the grid set to 1/8th notes, which has them on the correct places - it just highlights beat 1, 3, and 5 instead of 1 and 4.