I tip based on the service. Were they on time? Were the delivery men friendly? Did they set it up right? Did they take care around the rest of the furniture? Was the furniture in one piece (not broken)? If there was something to test, did they test it? What were the weather conditions? How big of a piece of furniture was it? Was the number of people right for the job? (ie: it doesn't take 4 men to deliver a lamp)
Then I tip between $2 and $20/person. The $20 went to the movers who moved our furniture from the apartment to the house, reassembled a couple of desks, were very friendly/courteous, very patient with rearranging where things went, had to deliver in the rain and still got things here and didn't damage anything... those guys were the only ones I've ever tipped $20, but they earned every penny. Usually, I end up tipping about $5/person. I've only *not* tipped a couple of times.