first- check to see how accurate your 1/4" seams are---cut 3 strips of fabric 6" x 1 1/2"---sew them together---measure the center strip- it should measure 1" wide- if it does not you should *adjust* your seam allowance until you have it correct.
you can add a smidge (consistant amount) to each piece of a block===but some blocks will simply not go together quite right- some are ok to do that to---then you need to measure & trim with each step of making the block. it is easier to figure out why they are coming out wrong & fix that- you may need to sew a bit narrower seams...or line your ruler up a little differently- like have the lines on the edge of the fabric---not beside the edge.