I tip 15-20% for wait staff in restaurants (except at fast food places where I give nothing). At a Chinese buffet, they get 10% as long as they keep my drink full and ask me if I like the food. I have only once ever had a reason to take a taxi. I probably just rounded up to the next dollar. At nice hotels, the staff can be aggressive about helping you with your bags in order to get a tip. If it is convenient, I bring in the bags later. If not, I firmly tell them to go away.
I hope you enjoy your trip.