If my quilt is measureing square to start with, I often just cut and sew and then trim the ends. If the quilt is not as square as I'd like, then I'll cut the borders the same and ease them in so that the sides are now the same length.
I will measure afterwards and rarely have any problems doing this, but I use to sew professionally, and when you sew 8 hours a day for several years, you get very good at knowing how to hold the fabric so it feeds correctly without pinning.
If I have a top that is divided into sections (like 12" blocks) I'll often lay the border on my mat and mark it every 12", but I am doing that more to make sure all the blocks stay 12" than to get the border correct.