Before you trim anything, please make sure that the 4 1/2" blocks were cut accurately, the seams are exactly 1/4 " and when you press, make sure nothing is folded. Ideally, if you follow your measurements and pressing correctly, you should have nothing to trim, or very little at the most. I always find than when my blocks are not size I planned, I either sewed too fast and missed the 1/4" or I didn't press well.
Maria