If they're really *piano* movers (not just movers of pianos), and they do a great, job, tip away. $10 a person, or more depending.
My movers (Duffy of Philadelphia) did an unbelievable great job, even moving other furniture to the basement for me, adjusting the area rug so the entire piano fit correctly, etc. If they had just "dropped it off", I wouldn't have tipped them.