It also depends on how they are cut to begin with. If by chance the fabric is at all cut even on the slightest bias they will stretch. I have noticed this when it comes to pressing. This is why I NEVER wash my fabrics before assembling my quilts. I use 100% cottons including the batting this way they all shrink together, and the starch that's in the fabric helps the blocks keep their shape.
It is a must to press after each step like previously posted.
Good luck and don't forget to post a picture when it's done!