See a similar thread I started below. According to Curt:
I think it's very important to run the reserve once in a while. Water, sediments, and crud tend to settle to the bottom of the fuel tank. If you never use reserve, this stuff will stay there. (Actually detergents in gasoline will take out some of the water, but I'll ignore that for the moment.) One day when you finally need to use the reserve it's full of all this junk, and your carb says, "Ooo, ick! What IS this stuff?" I think it's better to bleed it out a little at a time, so I think it's a good idea to run the tank pretty low (well into reserve.) It's also a good idea to install a fuel filter.