You haven't said what type of trim you will use or how it will be installed. Most casings are slightly hollowed out on the back and the two backside edges have about 1/2" that is proud of the hollowing. Drywall is seldom perfectly flush with windows and doors. It's typically caused by the framing members not being flush with each other. This hollowing on the back of the trim allows the casing to step over a small discrepancy and still fit tight to the window frame and the wall. An 1/8" is about the maximum it can step over, less is better. You can either cut the square edge of the drywall with a utility knife to bevel the edge so it's a bit softer, or some will tap the drywall and crush the edge with a hammer. The idea is to soften that sharp corner on the drywall.
You can take an average of the difference that the drywall sticks out and rip some strips to use as fillers, or extensions, to furr out the window frame so it's within a reasonable range for the casing to step over. In your case, that might be 3/16" strips. In more extreme circumstances, you may have to cut tapered furring strips.
If you are using flat boards for casings, it will be a big benefit to hollow out the backs with dado blades on a table saw or with a router bit in a router table. If you are picture framing around the window, 45s on all four corners, things may take a bit more work to get tight miters. This is because the trim might be twisted a little if the furring strips aren't exactly even with the drywall. Often undercutting or back beveling the 45s will solve this. You can also put on all 1/4" strips with brads, set the brads deep and use a block plane to shave the strips as necessary so they will be very close to being flush with the drywall.
If you are using a window sill and a headcasing, no miters, slight twists won't matter. You have a couple choices in attaching the furring strips. You can place them where you would want your casing reveal and keep the casing flush with them, or you could add a second reveal and hold the casings back from the edge of the strips, which I think looks better. Since I trim houses for a living, I don't mess with making all the furring strips nice and flush, too much time and you wouldn't know the difference when I'm done. I'll trim 20 windows in a day and start losing money if I don't, that's why I'll take an average for my furring or extension jambs. You may be able to take your time and do a bit more custom fitting. In either case, a hollow back will be your friend. If the faces don't come flush in the miters, you can shave a little off the proud edges on the back of one to bring things in line. An 1/8" deep hollow, which is more than standard moldings have, will give you some handy options when/if needed.
Normally I'm making extension jambs to furr out windows that are made for 2x4 construction in 2x6 framed walls. Same thing you have to do but I have wider pieces to work with. That way I can make a frame and install it as a unit. You'll have to nail on individual strips. Here are some pictures of a window with a sill and headcasing. I made the trim and hollowed out the backs. I can mass produce the pieces for the same size windows, all cut the same. I still have to fit the sills but the rest is a snap.
By the way, I like to have walls primed and painted, trim stained and one coat of finish, or primed and one coat of paint. This saves a ton of time compared to cutting in if things were to be painted or stained later. Nails on stained work will get filled with color putty and an additional top coat applied. Everything looks finished when installed this way.
Last edited by Hammer1; 02-08-2012 at 12:33 AM.