Squaring Blocks??

I've read a few places about squaring up your blocks prior to sewing the blocks together to make a quilt top. I've actually only done it a few times and mostly only on units that had triangles in them since I tend to make those a little "off".

My question to other quilters is how often do you square up your blocks? Do you square up the rows after you've put the blocks together into rows? Do you square up the top before you add borders? Do you square it up again after adding the borders?

I know that if you make a small mistake on every block, it will add up when you have a large quilt. I've only made lap sized quilts so far so the errors don't add up too much on the small quilts. I am however working on the queen sized quilt and now that I almost have all the rows together and I'm contemplating how to make the pieced borders, I'm getting a little concerned that my borders may not fit "just right" onto the quilt if I did make errors and didn't quite square up the blocks.

Any thoughts?