I always tip as long as they are careful & don't scuff the walls. We got a bedroom set 3 years ago & they had to lug 5 pieces of furniture up the stairs. 2 of the pieces were MASSIVE! They were very careful & pleasent. There were two of them so I gave them a 20.00 bill & asked them to split it.
A few months ago I got a livingroom set... (2 couches, and 4 tables) All of the items were small & light. They were in & out in about 15 min. I gave each of them a five.
To each is own I guess, but if they do good work & they can't be making all that much, giving a tip for good service makes me feel good!