Searing meat creates the Maillard reaction which gives a wonderful flavor that cannot be duplicated any other way. Searing in the beginning of a cook is commonly thought to "seal in the juices" of a piece of meat, however it has been proven that the opposite is actually true. By searing at the end you get the benefit of the flavors introduced by searing and the meat will stay juicy inside.
For tri-tip I like to smoke low until an internal temp of about 125. Then pull and foil loosely until the temp has peaked and then dropped back down to the 125 level. Then I sear over high heat until I get a nice crust. The internal rises about another 5 degrees. I let it rest again for at least 20 mins and slice just before serving.
2011 CBBQA Rookie ToY - 6th Overall, 2012 American Royal Invitational - Huminie's Hogalicious BBQ