If you are using purchased charms or cake layers...they often come not all the same size..off by 1/8" or so.Why, we dont' know! If you pre wash (which I do for this exact reason) they will shrink. I just washed some cake layers intending to cut them into 4, 5" squares. I got some that measured only 9,1/2 inches after the pre wash so they wouldn't cut into 4, 5"ers. Others in the same pre wash didn't shrink at all. So if I had not prewashed, the ones that shrank..would have shrank after the quilt was done. Some like that puckered look, other including me, don't.
If there is ANY stretch to the fabric...handle it gently. But on the other hand, use the stretch to your advantage. I always start by matching the two corners, then if there is any "give", I can even distribute it and a tad bit of stretch as I sew and the pieces go together just fine.
Even seam allowance, exact 1/4" all the time. The small the length you are sewing, the more exact it has to be. You can accomodate and work around a seam that is a tad bit crooked if the seam is 10+ inches long, but on a 5" seam length...not much there to forgive anything.
Press (not iron, as ironing will cause some stretch) before you cut the pieces. Then press and starch after you have cut them. That keeps the threads more secure while you sew.
Good luck and hope it works out!