Generally airline staff are not allowed to accept tips, so you donīt need to worry about this.

Unfortunately thereīs always an exception: Lounges

- most lounges in the US donīt offer comp. alcoholic beverages, on top of paying for your drink (cash or drink chit) bartenders will often expect a $1 tip
- some lounges have shower facilities and the "housekeeping"/cleaning staff tend to very much appreciate a tip (very poorly paid staff), some people will also pay a "tip" to avoid having to wait to use the showers

Apart from that I think the only occasion where tips are generally expected is when you need wheelchair assistance and the staff helps you with collecting your luggage, carrying it around etc.