I trim mine before sizing,otherwise they are a little to tight in my RCBS case trimmer. Sizing dies are all just a bit different in size. The difference between my 454 hornady die and my 454 RCBC die is considerably. The RCBC die being tighter, and about 60 fps faster over the hornady die loaded with same bullets and powder. I guess what I'm saying is there are many variables, and the only why to know for sure is try it both ways.