Depending on the company, most of these folks are independant contractors employed by the company you are hiring. The driver recieves a per mile payment, and sone form of start/stop payment. The diver has to cover fuel and a set aside towards maintenance, and cover the payment in most cases for purchase of his vehicle, three days is ten percent of the month so that is a substantial part of that months overhead. Their cost are substantial when you think about it. So if you recieve what you consider good service there is no reason not to offer a tip.
No I have never done this type of work, but I had a friend that did and quickly lost his arse when the jobs required 200 to 300 miles travel (unpaid) to get to the next pickup, and did not come up quick enough.