When I want an 8″ x 8″ block I want it to be exactly 8″ x 8″. I have found the best way to get a quilt block the size I want it, is to trim it down after it has been pieced.
If you are pressing in “bulk” (i.e. many quilt blocks at a time). Press one block then while it’s still warm, put it under a heavy book either on your ironing board or table.