Placing the fuel selector to left will not stop fuel moving from the right tank to the left tank via the vent line and then overboard out the vent, if the aircraft is parked on a slope with the left wing down.
Hmmm not sure about that BPF. I've observed the effects of
a) forgetting to select a single tank on parking (mucho fuel loss)
b) remembering (most of time after (a)), parked on a slope, no fuel loss.
All this aggravated by the FBO's habit of topping the tanks off at night.
But the 172R has a 'both' position. I believe some 172's don't so maybe they're different.