I think this is a very reasonable argument for per-project billing, for the reasons you mentioned. But the downside you mentioned is valid also - that some clie

I think this is a very reasonable argument for per-project billing, for the reasons you mentioned. But the downside you mentioned is valid also - that some clients will be greedy with your time. I prefer time based billing, because I don't work 8 hour days like normal people. For this reason I also make sure I'm working on things that can be done in those timeslots, and I'm very, very conscious of the client's time. I think either method is fine as long as you're comfortable with it, and you're on the same page as the client.