As a driver, my opinion on tips is they're appreciated, but by no means expected.

That being said, you as a passenger need to realize that if a driver is driving several miles across town for your pickup - that's their own gas they're burning, and not being paid anything. They're going out of their way to pick you up. If you're only going a short distance, the driver is losing money.

Please, for the love of all that's holy - if your driver is more than a couple of minutes away when they get your request - tip them. We could just as easily ignore your request, meaning you wait longer if you get a ride at all. The fact a driver is even willing to drive across town to pick you up deserves a tip.