I think that most issues have already been covered, but on my 3500S, when I bought it the reserve did not work, so local garage blew down the fuel line and cleared it. Suggested that 'crud' accumulated in the tank had blocked the inlet (lower in the tank)
Result was reserve working but later the symptoms of fuel starvation on normal setting. Crud blown from reserve was in he main fuel and blocked the in line filter. New filter cured all problems, but remember if the reserve inlet is blocked and blown clear, normal approach of a garage, then the muck has to go somewhere and will stay in the fuel tank and cause problems when is most inconvenient.
I suggest that if your reserve is working it does no harm to run the car on reserve once in a while. At least it stops the change -over tap from rusting in one position.