Standard Company policy in the US is that it is unacceptable to accept any gifts/tips from PAX. The logic behind the policy is that you are already paid to provide the service, therefor it is inappropriate to receive additional payment unsactioned by the company/airline. I've had passengers during Christmas time give the crews ipods and gift cards. Tipping is a big deal in the states - they tip for everything! Haircuts, taxis, waiters, doormen, curb-side checkin agents etc...