My Mum is in a wheelchair and practically deaf and when we travel (always with me in attendance) needs assistance such as the AmbuLift if there is no airbridge, getting from A to B in terminals and expediting getting through immigration and security.

We use assistance in:

Aberdeen (departure and arrival) - never tipped.
Transit in Heathrow T5 - never tipped.

Madrid, Malaga and Granada departures - never tipped. Why? I typically get her to the gate then they help from there; not so much for then to do, unless it's an non-airbridge boarding, when we need the van and ambulift. But haven't tipped then either.

Madrid, Malaga and Granada arrivals. I have tipped. Why? Plane to immigration (train if Madrid T4S), baggage claim and then exit can be very time consuming (not so much in Granada) and I cant handle a wheelchair, carry-on and checked luggage. Typically 5 Euros and always gratefully received.