Tipping is not only customary, it's probably a good idea. Not that agents typically do this to non-tippers, but I was in earshot of a few agents who were upset about a family not giving them a tip for checking six bags, and I saw a few bags get some harsh treatment when they were being loaded in the trolley to take inside.

I'd usually tip $3-5 if flying alone or with a companion, and a little more with a larger party.

+1. It's extremely easy to prevent this dilemma. Just carry the bags a little bit further and check in inside.