Do you have a personal relationship with that mechanic/shop, or is it just a place you drop the beast off for service like your car at quicklube? If your mechanic is giving you good service and taking care of your problems with attention to detail then a tip would not be out of line. Often its considered good form to show up at the LBS with a 6 pack of holiday cheer (or summertime cooling.........or whatever).

Check in advance to see how the mechanics share at the shop.