You are not worried that glue behind the sticker will yellow the print? I have seen stickers just fall right off after a decade or so and leave yellow mark.
As to signature looking badly, I really think it is just a matter of opinion. I went to a local municipal museum yesterday. I saw everything from unsigned photograph, signed ON the print in the image area, signed on borders, and signed on over mat. Strangely enough, I didn't see one float mounted and signed on the mount board. (my favorite method) A friend of mine who is an art history professor and an artist (painter) herself also says it is really up to the artist how he/she would sign (or not sign) his/her work.
Sharpie, when used on shiny surface, will eventually separate and leave a dark line surrounded by lighter area. I'd definitely NOT want to use that.
Anyway, thanks for chiming in.