When I first began quilting, I took a class from a professional, and we put dots on the fabric 1/4" in at all corners, then ran a pin through those dots. The results were beautiful. I'm just finishing a Lone Star and the templates in the book all had those dots. I matched them up carefully (every piece, all 288 of 'em!) and it worked perfectly.
Are you sewing bias edges to blocks cut on the straight of the goods? This can cause problems if you're not really careful.
Also, when you're pressing, just PRESS, don't iron by sliding the iron on your material. Even a little bit of the resultant stretching can cause problems.