I tend to bill on a daily basis. I will present my clients with my hourly rate and then explain to them that I bill per day instead. My daily rate is slightly slower than 8 times my hourly rate. The reason I do this is to prevent nitpicking on time sheets etc and to be accountable for every single hour of the day. The last thing I want to do is have a clock sitting next to me measuring my every single move. Some days I do some overtime (1 maybe to 2 hours max) and other days I find myself working slightly less. In the end it all works out fine. The estimates that I give my clients are based on days, not hours. If they want me to do a 4 hour job, cool, I still charge them for the entire day. Most clients are cool with that. Since most projects I do tend to have time-frames that go into weeks it's much easier that way for everyone involved.