If you have inset uppers, do you still need the 3" or so fill so the doors can open or does the face frame negate the need for that at the corners?
Face frame negates?
Yes. In the case of our cabinets the inset face frame creates the space needed so that you don't need a filler.
You can get away with just the stile if you have to.
I like to add a 1" using an extended ear in the corner (avoid fillers but have a good installer), gives a little breathing room in the corner and also allows for a nudge when installing.
Depends on brand and budget though. If paying for dimensional modifications and budget is tight then I might work it out to even up the sizing based on the standard wall cabinet depth. i.e. 13" deep add 2, 13.5 add 1.5, 14 add 1...Don't see deeper than that on a tight budget :)