While investigating for countries I found that Unicode defines several variants for plus and minus signs: http://en.wikipedia.org/wiki/Plus_and_minus_signs#Character_codes
Actually the dash we frequently use was always just a workaround. Apps that value good-looking typography (like Bitcoin Wallet) might want to prefer the full-width variants. I think that warrants the configurability of the minus sign. I always thought that '-' looks horrible and tried to enhance that with some narrow space inbetween. Now that I know of the variants I will try to use them.

I didn't know about the accounting number format. I've never seen it in any accounting app I ever used. Do you think we should support it?

btw. I also stumbled upon different variants of digits. For example in Europe you should use a '7' variant with a horizontal dash. But I think that goes too far for now.