I don't know, I would say keep on putting down the exact times you get there and leave (with other things like lunch, if something goes wrong, etc.) and then do the billing hours from that. That way you're covered in case a customer brings up any argument. Say you get there at 8:50 but started the clock at 8:45, even if you do adjust the ending time the customer may not notice that at first and your butt will be on the line.

The customer needs to be aware of how many hours each day that they were billed for, and if he questions anything then you bring out the sheet and say "hey, we got to your place at 9:00 and left at 1:00", they can't really say anything to that unless you spent an hour or so standing around goofing off.

Remember YOU'RE the installer, if the guy wants an in-wall speaker on an outside wall then you gotta point out up front that it's gonna take longer and requires more work IF he really wants it there. Haha, usually if I remind them that, "hey, we are charging $XXX an hour", they tend to see things differently. :)

Nick