Can you mix a miter with a regular straight?
I did the first border with a miter because of the stripes in it, so does the final border need to be mitered to look right? Or will it look just to funny to have 2 different corners?

I either miter or I don't but I have never mixed.

why not? As long as it is pleasing to you....like I read all the time here, there aren't any quilt police, so do what you like with your fabric. That would be like mitering an interior border and doing a seminole or some other pieced border on the outerone. I'd love to hear what more experienced quilters say.

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

What would the judges say at a quilt show?

I just happen to be struggling with this same dilemma - I have a striped inner border and a polka dot outer and would like to enter my quilt in our local quilt show. - I was told by an experienced quilter that the rule is 'mitering for stripes' because it gives a more pleasing effect.
Does it matter? Should I make them both mitered so that your eye will flow easier from the corner to the quilt design?
I'll definitely be watching this discussion- thanks Boxer Mom for posting the question.

In a show quilt I would be consistent in doing the borders all in one way.
However, it's your quilt. And there are no quilt police, despite what some would like you to think:)

If you haven't yet attached them to the quilt, it's really not any more difficult to do 2 fabrics at the same time.

And the end result is noticeable for the better, I think.

i believe it depends on the pattern in the border fabric. Some just look better mitred. However, having said that, I probably would not mix but do them the same to keep the look of the edge consistent.

i would miter all of them

I haven't been quilting long but at a class I took we were taught to sew your borders together and miter them all at the same time.