The idea of changing tip shape is to reduce induced drag - that is drag from the tip vortex generated in the production of lift.

in straight and level flight a racer is unlikely to be creating much tip vortex drag because lift co-efficients will be low - however during turns lift co-efficients will increase admittedly.

The variance in shape of tips in models outside the sphere of competition is governed by styling largely.

The likes of Andy Lennon claim an advantage in using diffuser (tip curving up) and retarder (tip curving down) wing tips as seen on certain full size aircraft.

I've heard a rounded tip is worse drag wise than a cut-off tip.

I had a trainer with the tip shape you mentioned with the bottom flat and angled up. Terrible tip stall into a spin at the drop of a hat.