Not at all. Imagine on your fuel tank there is 2 pipes coming out of the bottom and inside the tank one is flush with the bottom of the tank and one is sticking up 2 inches from the bottom. when the fuel is used to reach the top of the first pipe the bike cuts out, so you turn the tap and the valve selects the other pipe.
Running on the reserve position will not affect the way the bike runs, but if you run out of fuel then you have no option but to push it home or call the recovery people. Unless the tap is broken I wouldnt recommend it.
come and have a look at http://www.biketuna.co.uk/wiki for more maintenance info