It's easiest to add each border individually. Otherwise you can miscalculate length (don't ask me how I know that :oops: ). A lot. Plus, you don't have to miter the corners. Mitering corners can be quite difficult unless you have practiced it some.
When adding borders individually, first measure through the middle of the quilt in the *long* direction, cut your borders that length, and add them to the quilt. Then measure through the middle of the quilt in the *short* direction, cut borders to that length, and add them. This is the easiest and most efficient way to do it. Also, if you are cutting your borders along the lengthwise grain (along the selvedge edge) as many of us do, this will minimize the yardage needed for the border.