The rule for concierges is that you tip them for something you couldn't have done yourself. For example, if they make a reservation at a restaurant, you don't need to tip them. But if they use their personal contacts and influence to get you a table at a sold-out restaurant, a tip is appropriate.

Same for transportation arrangements. No need to tip for routine assistance, but you can tip to take advantage of his/her connections.

And I agree that tipping in the US is getting out of control, with the list of people to be tipped constantly growing and the standard amounts increasing. Don't feel pressured to tip anyone, even if they passive-aggressively ask for it.