It has become cruise folk lore that if you don't tip the porter at embarkation your bags won't make it aboard the ship. Not really true. In fact at Seattle our porter took our bags and carted them off before I could pull out my wallet, and never returned. Some will run their mouths giving a song-and-dance, but that is really all it is. They have too much luggage to handle to play games with individual passengers.
On the other hand, tipping a porter to take your luggage out of the terminal on debarkation day is a wise thing to do. They know the layout and can save you some time and effort. Plus they are giving truly individual service as opposed to what happens on embarkation day.
If you ever notice when a bus pulls up at the pier from the airport, they offload the luggage before the passengers are allowed off. Some will fearfully tip anyway, but many just walk off to check-in. I've never heard of anyone's luggage not making it aboard.