I usually make my binding with the same fabric as my last border, but I also do pieced borders and pieced bindings. When buying fabrics for a quilt, I usually round up to the next whole yard (if it says 5/8th, I buy 1 yard) to take care of the oops! factor, so I always have extra.
I cut my binding WOF. The only time I used a bias binding was when the fabric I chose was striped, and it gave the binding a candy cane look, which was great, because it was a child's quilt. I've not done any curved quilts, so haven't had to make bias binding for one.