| In some cases, the thing to do is to add an additional extension jamb onto the window that will be an appropriate thickness (like 1/4 or 3/8") so that the window jamb is flush with the wall in most places, recessed in some place and maybe proud in a few- in other words you have to split the difference. You would not make it variable thickness, you'd pick one measurement and make it all the same- If needed you can mud around the window where the new jamb extension is sticking out past the wall so as to level the wall surface out with the extension jamb. Once you do that, the wall will be perfectly flush with your new 3/8" piece that you added, and your casing will lay flatter on the wall. IMO, if a jamb is recessed behind the drywall no more than 1/8", that's perfect. When the jamb is proud, your miters open up and it's more difficult to get them to match.
In other cases, the window hasn't been installed straight with the wall due to the flexibility in the nailing flange-you might be able to move the window in or out a little, provided it isn't foamed in place... then run a trim screw through the window jamb to hold it in place. This is probably the first thing you should check if your window has a nailing flange- see if you can pull the window in, or push it out any- it might help.
Most trim carpenters have to compensate for the unevenness of the wall by tipping the casing, beating the drywall down, and creating a compound miter. Professionals have no problem getting these miters to line up, but I can imagine it might not be as easy for a DIYer.  |