I think it depends on the country. In here Finland, one mobile operator offers those stickers for free. Then user can add certain amount of money to sticker using their online bank. After money is added, user can use the sticker to pay for stuff in normal stores. And really - this is completely free for the user, no any monthly fees etc.
Over the camera would be a poor location for it. Aside from that anywhere should be fine, those stickers won't be able to block wifi/cellular/GPS antennas/radios in the iPhones design. If you are a really particular person goto ifixit.com and look at pics of your iPhone and try to put it over the battery as much as possible. Regardless the sticker isn't going to block anything more then the housing the phone is in to begin with.