Its up to you. Personally I wouldn't unless it was their actual birthday, but I know many would go ahead. They don't verify birthdays, so getting the sticker wouldn't be a problem.
Adding more info! Another reason we wouldn't get the birthday sticker on a day that wasn't our actual birthday is my very precocious 5 year old would loudly proclaim that it isn't their birthday if anyone tried to say Happy Birthday to them!