In Baltimore the word 'tip' has never been uttered when we were cruising from there. Of course you pull right up to the luggage trailers they use and it is almost self-serve although they make sure you have tags on the bags and aren't any help if you don't. "Go to that tent at the end of the parking lot" is what they said and you have to go there to get luggage tags if you don't have any. In our case we'd been upgraded to a different cabin so our tags were no good. So, off I went to the tent, came back and tagged the bags, and tossed them on the trailer and went inside. Thus, no tips were offered and they didn't seem to care one way or another. They weren't jerks, just aloof.
In Seattle once, we had a lady porter and she took the bags with a smile and walked away before any tip could be offered. She looked out of place and I got the sense she didn't normally handle luggage.