Fuel tanks are not very deep, there cannot be a significant increase in pressure feed down the fuel line.
I always thought that you select the fullest tank if there is an imbalance simply to bring the aircraft back into balance, why select the other tank and make the balance issue worse?
On a 172 with L,R, Both and off settings the fuel will crossfeed in both and off but not in L or R. So if you are flying out of balance with Both selected then you will end up with an imbalance and as has already been mentioned if you park on a sideways slope then fuel will fill the lower tank if Both or Off is selected. Expensive if it is left wing down!