I go bigger, then trim. If you want a 4.5" square and then do a 4 7/8 with 1/4" seams, right off the bat aren't you pulling it in MORE than 1/2 inch? Because of the angle?
Anyhoo, I'm just not that accurate. I'd rather sew, sew, sew and then trim, trim, trim. I've tried to do it the other way and ended up sew, sew, sewing some more...