I miter mine, and use a grout that closely matches the tile. Never heard a complaint.
Truth be told I like doing it with a pencil but it seems a lot of people don't want to draw attention to the niche.
I'm not saying it shouldn't or can't be done, but it just really hard to get it to look good. Got any pics?
Also, just a bit of advice, don't think because you are not hearing it, that they are not complaining. Most people complain to everyone but the one they should complain to.
I agree that we should try to make them as least obtrusive as possible, but I don't have many not wanting accent tiles and nice trim around them. It's a chance to make something utilitarian by nature look nice.
A pencil or chair rail piece can increase the depth of the shelf from 3/4"-1 1/2". So that usually overcomes any hesitation on bringing attention to the niche.
EDIT: I miter mine as well, I still just don't like the look. The corners pull in to the niche. I like a sharp crisp corner. Bullnose on the outside or pencil/chair rail on the outside. I have done some chipped edge natural stone that allowed me to chip the corners to perfection, but that has been far and few between.