Well i'm going to take the firm stance that these guys don't get tipped.

I don't tip the pilot
I don't tip the FA
I don't tip the check in people

The US tipping culture really grates and you end up with a system whereby the service element gets stripped out of the product.

FFS - why tip a hotel chambermaid on a routine basis. You're paying $$$ for the room, and unless I missed it in the small print that includes having the beds made and towels changed,

You then end up with a situation where people are expecting a tip for doing the bear minimum and a bigger tip for being 'good'.

Must have used the EK Chauffeur service 20 times across the globe. Never tipped once and never got any hard feelings from the driver.

(sorry for the rant)