Theres nothing wrong with flat end wingtips. However, your wingtip blocks you want to omit maybe what is strengthening that area of the wing. If you carve and sand them close to the same shape that will be fine its not all that critical on a model aircraft.

To further the discussion a bit more different wingtip configurations can offer some interesting aerodynamic features. For example the winglets on airliners improve fuel efficiency. Wingtip fences on a model airplane will increase roll rate and delay stalling because they delay boundary layer separation to some degree.