Cessna 172/182: Should I set the tank selector to the R.H. tank after parking the airplane? The single-engine Cessna 172 and 182 aircraft models have a dual gravity-fed fuel system, where you can draw from both the LH and RH tank simultaneously in flight.

I've heard it said that when you are securing the airplane after a flight to leave the tank selector on the RH tank to prevent crossflow if the ramp is uneven. (So you don't end up with all the fuel in one tank).

Is there any merit to this?  Does it actually block the crossflow valve to do this?
The C172S manual I have says to set the selector to left or right as part of the "securing airplane" checklist:


  
  Fuel Selector Valve -- LEFT or RIGHT to prevent cross feeding
  


I was also taught to set it to right during my initial training. The idea is to prevent cross-contamination or leaks if one tank is compromised. Another reason is that if you park the aircraft on a slope, you could end up with an imbalance between the tanks, as you said. 

So the POH confirms that it prevents cross feeding, at least in that model: you should always check the POH for your specific aircraft.