I don't know the Supercub at all, but the general idea is that the checklist should be laid out so that you initially startup and taxi out on the less full tank and then (after a period of time during which, unavoidably, the entire volume of fuel in the pipe from that tank must have shifted) you change to the fuller tank and then during the power tests you unavoidably shift the entire volume of fuel in the pipe from that tank.
To do otherwise means you could be taking off with a tank selected, whose delivery has not been verified.
Even if one does a very short taxi before the power checks (too short to check that tank) one should easily use up enough fuel in the power checks to check out the tank used for takeoff. Does this make sense?
I suppose if the power checks were done very quickly...