I recently got a few Japanese 35mm cameras and lenses with the stickers. Back in the day I used only German equipment, so no sticker. Also, I currently have about 15 Japanese large format lenses and none of them has the sticker.
Yeah, you want to piss off a Leicaphool put one of those stickers on a Leica!
I leave them on if they're not about to fall off, though when they're on the focusing ring of a lens (often) it's annoying. Call me an amateur -- I am! If they're long gone and the residue is there, I'll use naphtha to try and remove it. Interesting question as to whether collectors regard their presence as a sign of a well-cared-for piece of equipment.