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From the wall street journal online:
"Etiquette and tipping experts agree that tipping a contractor and his employees isn't expected because contractors offer to do a job for you at a specific price. Any extra money they would want would be built into their bid. Also, it's a business relationship rather than one where they are performing a personal service for you like a waiter or a maid. But if the employees do extra jobs around the house, then experts say it is appropriate to tip a cash amount equivalent to the task; "tip gifts" such as cookies and drinks can count. "The key to tipping is whether or not it was outside the scope of what was normally expected," says Mark Brenner, author of "Tipping for Success!"
My opinion is that a little bit of courtesy (snacks, drinks, expression of appreciation etc.) goes a long way and upon completion of their work you feel compelled by a job well done to offer a little extra in the form of a tip - go for it, but do not feel obligated.