Mutt.
My employer requires us to spend our own money on trips and we are reimbursed including tips spent. There is even a "Line Service Tips" category on the expense report. It's to be expected in the US.
At least $40 on arrival and occasionally $100 if we need special attention or "hangar is full"....sometimes the hangar is not so full. On the GLEX everything costs more....including tipping.
Also, this helps us on return visits as the guys remember.