The amount you give in tips does vary according to how good you feel the service has been.
They normally expect 15 -20% as a tip. We didn't tip the hotel receptionist, but it is usual to give a couple of dollars to the doorman if he hails you a cab, opens the door etc. If a bell boy takes your cases to the room, then he expects 1 or 2 dollars a case. From memory, I think I gave the cab driver a $10 tip on top of the total $50 fare inclusive of tolls.
In bars and restaurants, even if sitting up at the bar, they still look for a tip. Many restaurants do add the gratuity to the bill, but the waiter will still ask if you want change if paying cash - it happened to us a couple of times.
One further hint, if you've not been to NY before. The cab drivers are usually notorious for not knowing where they are going, as many of them are recent immigrants. With a few exceptions (such as the Waldorf Hotel), they find their way around by the junctions of the avenues and streets. For example, if you wanted Tiffany's, it's as well to say 'corner 57th and 5th Avenue'. If you have a reasonable map, you'll be able to identify where you want to be dropped. It just makes life a lot easier.
I hope this helps, if you want to know anything else, just ask.
John