Once flew for a while with one fuel cap off in a C172.
[Pause for inevitable insults.]
Putting the tank selector to one tank prevented all the fuel from both tanks from siphoning out of the open cap.
Happily, I lost no fuel apart from normal burn-off.
On another note, is the on-off valve of a 152 not downstream of the left-right tank junction? So selecting fuel off will not prevent fuel from going from upper to lower when parked on a slope.
O8