If you're going to tip the wheelchair helper then why not the person at the check-in desk? The security people? The lounge dragon? The gate agent who smiles and wishes you a good flight?

I realise that in the US you are expected to shower money upon anyone and everyone you encounter in the course of your day but, even there, I would consider this to be one of the services that an airport provides and that you have already paid for (through the nose) in airport fees.