I think "don't double book yourself" is a good rule of thumb, especially when you're getting your sea legs, but there are ways to do that, with daily (or weekly, or monthly) billing, once you know what you're doing and you're confident about your scheduling and your business relationships and the way you set expectations.

The one expectation you should never set with your clients is hour-by-hour accountability. It's not even a win for the client: it just creates anxiety.