Usually, if there are multiple borders (stripes, solids, prints, whatever), the separate fabrics would be sewn together, then attached to the quilt, resulting in ONE continuous miter.
There is a bit of matching involved as you make the miter (at each fabric change, plus the stripes, if there are any), but it's a much more elegant, finished look.
If you've already attached the first border, I would continue with putting miters on the rest of the borders (but definitely join them if there will be more than one additional - making sure it's long enough to the miter on the final size). It sometimes requires a bit of adjusting to get a miter to line up exactly with a previous miter. Ask me how I know this.:roll:
I suppose you could use cornerstones as a way to get around it. But then it's your quilt. You can do whatever you want. :-D