BankToTurn,
Actually, a rounded tip like you describe is one of the highest drag tips you can put on, due to the wingtip vortex. The simple squared off "last rib" tip is actually one of the more efficient.

The idea of leaving the ailerons "short" so they don't reach the wingtip to reduce their effectiveness in level flight while still allowing a lot of area to be in the prop blast is an intersting one. Though you could get the same effect by tapering the ailerons as well.