They CAN miter the edges:

But I prefer not to. It makes for a very weak corner that can easily be chipped. I much prefer to cut the pieces to size, and then bullnose the edges of the stone.

If you REALLY want to get fancy, you can always use chair rail to trim the edges:

If they can't, the bullnosing can be done by any stone shop (read that as countertop place)

Bill's said it all, pretty much, but just want to make sure the obvious hasn't been overlooked... Maybe your tile has a matching bullnose piece already available. Did you check?

Thanks BV for the idea of using chair rail around the niche, I hadn't thought about that. Hmmm, must consider.

Here's another possibility. (The camera lens makes it look bowed out, but it really is square!)

My tile setter used the bullnose pieces from the matching tile and pieces of marble.

The center marble shelf is two pieces of marble placed together back to back to make it double thickness) and then a small strip cut to go across the front and hide the seam.

Then he used the same marble for the top of the little seat/foot rest.