You may find an occasional combination that's too tight or is looser than you like. For the loose fit, there are little rubber thingies that tension the rear pin making the fit feel tighter. For that occasional "too tight" fit, fortunately it seldom happens. I've seen some filing done on the upper but I don't condone this solution.
I was doing some mix-n-match a couple of weeks ago trying to find exactly what my older Son was looking for in a hunting carbine. When he'd picked the lower with the trigger feel he liked and the upper with the features he wanted, guess what, the fit was tighter than I felt comfortable with. A simple 5 minute job to swap fire control parts to another lower but had there been only one lower available, that would have been unhandy.