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.

That's one of those lessons that I will keep in mind when working with pre-cuts.

Very beautiful quilt!

The reason you stated is exactly why I don't buy pre-cuts any more. I bought a jelly roll, some of the strips were 2.5", others were 2.25", not usable for the project I wanted to use them in.