My seams are "close enough" so that the unit or block ends up to be approximately the size I'm trying for.
My tolerance - after pressing - is about plus or minus 1/8 of an inch.
Perfect? No. Is the end result satisfactory? Usually.
Some fabrics are easier to handle than others.
It's a bit like learning to drive a car - after a while one learns where one is in relation ro the road, etc.
Practice and knowing one's machine also help.