Gear up or down is case by case, there's no blanket rule. Verify that the aft CG limit is not exceeded in whichever configuration places the CG most aft. In the case of the Corsair, this is gear up and no fuel. With full fuel and gear the down the CG will move forward, this is normal.

In the case of the Hangar 9 Corsair 50 it is recommended by Horizon Hobby that it be balanced gear up.

Craig