Gentlemen and Ladies
Consider this especially if you work for a smaller airline flying to smaller cities on a regular basis. The people driving these vans get to know you Very Well. Tipping can sometimes make the difference in hanging outside the airport for a short amount of time or for what may seem like hours.
I have tipped when I was s 15k a year f/o at a commuter, I have tipped as a CA at a cargo airline, all for the same result, better service next time I get to town. And I am a "Thrifty" guy, but I am not a cheapskate. If one of my crew does not have the cash I come up with it, again so we get prompt service next time we arrive.
Tipping is indeed a personal choice, Crappy service don't tip. Good service, Tip and it may come back around to serve you well someday!