No actual airline employees (ticket/gate agents, AAngels, FAs, pilots, ramp crews) should be tipped with cash. They are professionals who receive salaries, and as noted above, they might be quite offended if you try to slip them cash. If you want to offer a special thanks to your flight attendants, give them a token gift--box of cookies/chocolates, Starbucks card, etc, along with a promise to write a nice note to their supervisor about their great work. I have done so a few times and they are always very appreciative. But never cash.
However, it is worth saying in this thread you SHOULD tip contract service workers (bartenders, janitorial staff who pick up your glasses in the lounges, shower attendants, etc). They earn very low wages from the subcontractor and are absolutely appropriate to tip in cash for the type of work they do.