I usually set my seam guide to a very scant 1/4"...basically, that's about a thread's width away from a perfect1/4". However, RARELY, IF EVER do I sew a perfect seam every time. I mean, really, with only a thread's width of tolerance, how could I be THAT perfect? (I tell my DH and kids that I'm, "perfect," but they all know that it's a snow job.) And what's more, does it really matter? So long as your blocks meet the size requirements, square up and match up with the other blocks, you should be fine.
~ Cindy