Some a/c, Rans S6 with two wing tanks have Left, Right & Off tap.
So the choice is easy, fullest tank.
Despite this separation the low wing overflow syndrome is still possible when parked left wing low - as in my shed - and Right selected. Because the pumped fuel return bleed is to the left tank, a surprising amount can drain from the 'top' right tank via the return bleed in a week parked.
I know !
mike hallam