In my experience people have all kinds of expectations, regardless of wether you charge them or not. And billing shows them your time is as valuable as theirs. On the other hand, if you offer your time for free, people will tend to load you with as much work as possible because, hey, you're free; and you have no way to leverage how much you're willing to do.
I figure with free, you've always got the opportunity to say "Hey, that's as much as I can afford to do for free" in response to unreasonable requests. With cheap, you then have to deal with the added "but I paid you money" argument as well. I'm sure it differs on how the job is presented and how you outline your responsibilities as well.
