I agree that there's no problem at all with asking and that offering to pay is quite appropriate. Even if you are talking all of $5 or less, many do not realize that, so it's better to be polite than assume. Do ask well in advance, though (i.e., when you are planning your trip). Asking once you arrive and have your charging cord out isn't the way to do it.
Try to depend as little as possible on their charging, too, and show up with the gear you need. That's just being a good guest. Of course, all this depends on what your host offers and what sort of relationship you have.
We actually had something of a situation like this over Thanksgiving, driving to Saint Louis and back. We were staying with my wife's great-aunt. She's a very gracious hostess, and she offered to let us charge at her home. She only has a 120V outlet, but that was more than enough over the few days we spent with her. I could tell, though, that she was concerned about what she was getting into. We assured her that it probably would cost on the order of $5 and that we were bringing our own extension cord. She was totally cool with it and even engaged in some good-natured bragging about it all (despite being a lifelong GM employee).
Grace begets grace.