My 3 cents are as follows - In US properties, even if my room is reserved for ONE, I am often asked how many keys I want. I don't think its that big of a deal for US or Canadian properties if an accidental extra guest appears. For overseas properties, you may have a problem. I'd want to be sure that everyone is a registered guest, even if it results in an extra charge. Hotel security and verification of ID of the guests seems to be quite strict in some locations, which is fine with me (which may include the copying of passports). And you should expect paying more for the exact same room with 2 people in Europe (as opposed to North American properties which generally charge the same for 1 or 2, as William says).