Somewhere on that plaster guard, there should be min/max lines. If the finished wall is between those two then the trim will fit. Some people like less projecting out, so you may want to go towards one end of the spec. My guess is with the plaster guard flush, you are near one end...if the wall ends up a little thicker, the trim won't fit without buying an extension kit.
If it sticks too far out, there's no way to fix it except to reset the valve which may be tough if you don't have access. If it is too far in, you may need to purchase an extension kit (if even available), which may look funky. So, look for the min/max lines, and judge accordingly to get it somewhere in between.
Most people prefer the looks if it isn't projecting into the shower near the max.