I think it would be appropriate. I have signed several of my pieces, including a mini sculpture of a Protectron robot from the game Fallout 3. I don't think by signing it, you're saying "I own the rights to this image" but "I made this." And I do agree that it should be small and on the bottom. I don't know about doing it on jewelry, but on figurines, sculptures, etc. I think it's appropriate.
Going to go check out your shop! :)