While I think you can operate it either way, and have seen it doen as both, and have rented equipment based on equipment hours as well, Yes, I agree, I would do it on time on site hours, and I would give them 15 minutes to 30 minutes on both sides of the 8 to insure them I was not trying to cheat them on a time job.
That said, my point was, and I am guessing from the questions that Rutwad is new at this, that I would make how the time is kept, very clear before the work started.
I would not like to see him in the situation where he worked his butt off all week, comes to the end of the week and feels he has done a fantastic job for this guy, tells the guy he has in his 40 hours, the guy walks over to the machine, looks at the meter and say's, Nope, you only have 25 hours since you started.
It is always better to identify potential problems before they are problems, then to get to the end.
As Fishpick I think it was said too, you will often run into folks that the job will not work out. Best thing to do is as stated, smile, move on, appreciate the chance to bid, hope you will consider us for your future work.
Other tidbits to the new business guy, when you get that feeling in your gut that this deal is not going well, don't get into it. Very seldom (I think never in our case) does it get better.
Set your price. Don't let the customer set it. Only you, and maybe your accountant, know what it is worth to you to do a particular job. If you start folding on your prices, and letting others dictate what they will pay, you will find yourself in financial trouble in the business pretty quick.
Gregg Whitstock the pond guy has a great way of dropping his prices (as people like to get a "deal")
Make sure you add in some extra's to the deal. Not sure what that would be in your instance, but say, in building a pond, an extra line would be night lighting. Then when folks go, oh, that is too expensive, you can offer to back the lighting out of the bid. Most times, they will go ahead and leave it in, but it shows the customer that you are willing to work with them on price, while not compromising your basic pricing structure.