I usually make a couple of test strips and will adjust my seam allowance until I get the size. Then I always make a few extra. I will remeasure the units periodically as I stitch especially if I have stitched for a while or if I had to change to different machine. I really don't like sliver trimming at all. I usually do press my blocks as I go and I figure that the pressing process sometimes stretches the fabric a bit so if I have to trim it down, I may end up a bit short. I think of fabric as a fluid media in that it does have some give and sometimes does distort a bit but will sometimes distort itself back when it gets washed or handled again. So, accurate cutting, not over pressing or handling the fabric, and making sure my seam is right, is really important. No, I don't always get perfect points or size, but generally, it really is close enough so the quilt looks just fine when finally done.