Joe, you are giving me credit than you should. <VBG> In NYC they are most likly union and I have 'heard' they will drop, lose, or otherwise do something to your bag if you don't tip. In Miami and other Florida ports I believe they work for another company and are basically independant contractors and tips are the main source of income. They are also not going to toss your bag into the drink if you don't tip them but it isn't something that I think one should do. Most are pretty good guys and work to help you.
true story, when we arrived in NYC onboard the CCL Legend I was gathering my luggage to wheel out and meet some friends that were picking us up. I have one of those foldup luggage carriers and did not need a porter. A porter approached my wife and told her she 'better' use a porter like him or 'something' could go wrong. When she told me I quickly told the porter if anything happened to my luggage or wife when I wasn't looking I would hold him personally responsible and not in a nice way. He left but not without tossing more threats over his shoulder as he got on the elevator. I would not trust them at that terminal.
In Tampa we had our entire family including my elder mother going on a cruise and the 13 of us each had a carry-on and one bag of luggage. (pre9/11 BTW) We decieded to just carry our luggage onboard with us. After I in the lead had reached the top of the escalator in the terminal a porter stopped my wife and mother who were behind me and demanded they give him all the luggage and tip $2 per bag and refused to let them pass until they did so stating it was not permissable to carry your own luggage on the ship. He was literally screaming at them. I went back down the escalator the wrong way and removed him from their path and called a cop myself after he threatened me with calling security. <VBG> He lost his job and if I had had time he would have been arrested for simple assault for threatening and scaring my mother but no time to go downtown and press charges. Bottom line, treat the porters with respect and tip but also do not take any guff off them, they are working for you.