I'm a lurker but I just wanted to add my 2 cents.
My DH and I grow and sell Christmas trees and have 3 lots, 2 in the DC metro area. While there is tipping that occurs, it isn't necessary. (our trees start at $50 for a 6 footer and go up to $450 for a 14 footer)
Some customers tip, some don't, but they all receive the same treatment. Regardless, we pay our employees more than minimum wage. When they are tipped, they are very gracious but they NEVER expect a tip, ever!
That being said, it isn't unusual for someone to tip our guys with other things than just money. Sometimes, it's a good bottle of wine or a gift certificate to a local restruant, and we've even had a customer buy everyone on the lot lunch before.(we have a very loyal customer base and a loyal group of seasonal employees, so there are always lots of familiar faces) Last year, for instance, on the lot at our farm, I had a customer bring me a rib roast for our Christmas dinner because I went out of my way to get him the perfect tree. This year, he brought us all hot chocolate when he came to buy his tree.