Since you said that your 1/4" seams are good, I'd also check your pressing. That was my biggest problem - I tend to IRON instead of press, and things get really wonky when I do that.
I bought some "Strip Sticks" -
http://www.thestripstick.com/ and those have been a game-changer for me. It improved so MANY areas of my quilting that I would give up my rotary cutter before I would give up my Strip Sticks!!
And BTW... once you finish your quilt, nobody else will ever know where the mistake was at when they look at it.