We tip $1-2 per bag for each handler. I can really only vouch for HAL, but I believe at most piers, there is a sign that handlers are salaried and to please report any overt solicitations. I approach the pier handler with his tip in my hand, so he already knows there's a tip and needn't ask. I have heard (not witnessed) that those who refuse to tip have their bags put to the side and are the last to be loaded. We always arrive at the pier around noon, and have received our luggage anywhere from 2:00 to 4:00 (and they don't all come at the same time, either), so I don't think it matters.
Candy <-- 11 days 'til our 5th Zuiderdam cruise