Nobody's going to like my answer, but I never square up my blocks. But I am a perfectionist - all my corners have to match and my points have to be sharp. So my advice is to learn to be very precise with your cutting (whether you use a ruler or a template) and VERY accurate with your 1/4 inch seam width and be careful not to stretch your fabric while you sew or your blocks can get skewed that way too. If something isn't perfect at the block stage, I tear it out right then and redo it so there are no surprises when I start putting the whole quilt together. Sure you sacrifice speed for accuracy but, in the end, you make it up by not having to spend a lot of time adjusting and tweaking your blocks to get them to fit.
If you've already sewn the blocks, you'll have to square them. But I agree with MaryQC, make sure there aren't some individual blocks that aren't quite right and fix them if you can. You'll be so much happier with the results.