I generally cut my top and bottom borders selvage to selvage and if I have plenty of fabric I would cut side borders the length of fabric.
One thing that really be an issue is when a client has constructed the design area of the quilt without paying much attention to making sure all seams meet correctly etc resulting in fullness and other issues and then cuts the borders from the length of fabric. Now there is a bunch of fullness that has become trapped by the borders that have little or no "give".
It's quite a challenge to tame the problem areas but so far I've made out ok.