You have probably been lucky! The problem is that you can stretch the quilt, and make the border bigger than it should be. This results in wavy borders that can't be 'quilted out'. It's more of a problem with larger quilts, so if you are doing smaller ones, it's less of an issue. the border should be pinned the entire length before you sew it.
As far as 4 Dog's question, I would measure the quilt, then transfer that measurement to the piece you cut for the border, leaving extra fabric at the start & end. You can then pin the correct length of border to the quilt, and still have extra for the miters.