Am I the only one who saw this heading, and thought of people tipping over wheelchairs, a la "cow tipping"? 😆
Seriously, there have been times I've availed myself of the wheelchairs at the airport, usually if there is a major distance in a connection (I am ambulatory, but not fast). Usually, I've done $5 to $10, depending upon how much they had to hustle.
The one time I tipped big was when I came back from Europe, and my knee had totally wonked out on the last day. At IAD, the wheelchair guy took care of me for over an hour, waiting for the one customs guy -- he was going through the bags of the guy in front of me very thorouoghly. I tipped him $30, mostly because of the time involved.