The original poster noted he was flying a C172S. This model has the fuel injected Lycoming IO360, therefore there are considerable differences between this aircraft and the earlier carb equiped lycoming powered C 172's (1968 to 1985) and care should be taken when applying procedures applicable to the old models to the new build (1998 to present) varients.
To answer the original question, the fuel selector should be placed on left or right particularly if the the parkign surface results in the aircraft tilted to the left as fuel will migrate to the low wing and then out the fuel vent. I takes very little slope for this to happen if the tanks are full.
It is also a good idea because it exercises the fuel valve. I was in one privately operated C172 whose owner always left the valve in the "both position" It was so stiff I had real doubts about getting it to the off position without breaking it....not a good thing in the case of an engine fire or forced approach