I have a 1/4" foot for my machine and, when I used it, I noticed that my blocks turned out slightly smaller than they should. Then I read something by Bonnie Hunter that said you should line up your fabric so that it is a hair over the line before cutting it (if you're right handed like me, it should stick out just slightly to the left of the line). Ever since I started doing that, my blocks end up (mostly) the right size. I also have been making things like half-square triangles a little bigger and then cutting them down to size (still watching that line on the left). That's worked for me. So for me, it's not my seam allowance that's the problem, but rather my cutting.