My husband uses a wheelchair (pink-tagged at the gate), and the tip depends on the service. Back when he used a manual chair, the trip from ticket counter to gate was worth five bucks. Gate to plane was a couple of bucks, with extra if they didn't f--- up the transfer from straightback to seat on plane. Trip from plane to baggage claim was five, again with extra for not scraping his butt on the transfer. One time a pusher took us to the parking deck and made sure we were properly in our car. That was fifteen-dollar service!

Now that he uses a power chair, things are more straightforward. He can propel himself to and from the gate, so we really don't need a lot of extra services. The tip is mostly based on the transfers to/from the straightback. Do it well and you are rewarded. Make things uncomfortable or embarrassing, then not so much.