I did this on an addition in my old house in Md (late 70s) and I would not do it again. I learned a lot of lessons on that project
The reason you have to trim the panels is they are actually 2x4 and the grid is 2x4 on center. You are trimming off the thickness of the T bar. YMMV it seems, some seem to be a tad smaller than 2x4 and go in without trimming.
