Thanks everyone, for the info :-)

Yeah, I guess it's one part laziness on my part - for instance, I'm working midnites tonight (7p-7a), and when I get home tomorrow morning, I want to just throw the bike on the rack and take it to the local park... without having to also re-hook the rack back up. As for mileage, I don't drive too far to work (20 minutes) and my little Toyota Yaris gets 40mpg... :-) I guess the solution will be to leave it on if I know I'm going to use it again in a day/two... but take it off if it's going to be longer than that.