15% in restaurants is considered more or less mandatory for sufficient service, 20% maximum if it is exceptional, 10% if the service was downright bad; Bellmen/Porters $2/bag; Taxis 10% of fare; Valets $2. I would NEVER attempt to tip a desk clerk in a respectable hotel, but I always ask for an upgrade
Edited to add: You are not expected to tip on the tax of your restaurant bill, so you may just tip the 15% of the subtotal before the tax is added at the bottom of the bill.