I too miter my borders frequently--- I do join multiple borders together- then add them as one unit to the quilt- my miters come out much better this way- just press 2 of them in one direction- then the other 2 in the opposite direction- the ones added to the sides pressed in one direction- the borders added to the top & bottom the opposite direction- then when you fold them together to form your miter you can nest the seams -- they come out perfectly- and much less work.
a great way to go!