A tip is an 'extra...not to be expected per se. Yes, most drivers will expect a tip, but that doesn't always mean they should get one. I tend to tip accordingly. If I get so-so service, I hand over a so-so tip. But, those drivers who greet me enthusiastically and then grab my bags and stow them for me, open doors, provide small talk if I seem to want small talk, or no talk if I don't engage, then pull my bags out at my destination...then yes, they will be tipped at 20%.
But, if that driver isn't where he/she is supposed to be and I have to go looking for them, bags in hand, or if they are morose and cranky and have nothing to say.....pretty much leaving baggage handling to me and then just driving me to my destination...then, they 'may' get a few dollars.
I tip on each leg of the trip...either 10% of the whole trip, or 20% of each leg. That way, if I get different drivers, no one gets stiffed.
I have to agree with Ed on this one...no driver should feel automatically entitled to a tip. Give me good service and I'll tip you. Otherwise, my fee has paid for my ride. It's up to the drivers employer to pay him/her. I will not augment someone's pay with tips simply because they expect me to.
Show up in a clean, fresh-smelling car, stow my bags, drive safely to my destination, chat with me if I'm inclined, or don't if I seem to want to be quiet, happily make a stop for me if that is my wish and you knew about it. Be on-time, where you are supposed to be. My wants are simple....provide what I expect and you will be tipped accordingly. But, if I have to go in search of you, your car is musty or smoke filled, I have to hoist my bags into the trunk and then you talk on your cellphone the whole way, no, you'll be getting much less than you are expecting.