Well winglets of the type that are there to make the tip vortex work to lower drag in a straight and level flight aren't going to do much for you in an aerobatic model so it is reasonably correct to say they aren't too useful. I am speaking of the nicely curved kind that hang down under the wing tip.

What Dick shows works fine for both upright and inverted to keep the tip flow nice as well as working in the SFG mode. You don't have to be in knife edge, you can get the effect in a side slip maneuver. If you calculate the total force it isn't much though. The fuselage is producing most of the side force.

They would be appropriate to put on the wing at around the semi span point also. It would stop span wise flow at high angles of attack. Of course the down side is you would be knocking them off all the time while handling the airplane.

Here is an example of dual SFG on an airplane from a gentleman from Mexico at the last XFC contest. I forget his name right now. His were removable. He set up this configuration to work at the altitudes of Mexico City and at the altitude of Indiana it would do nice knife edge square loops. It reminded me of one of the little foamy indoor models.

The change in chord forward at the tip was an attempt to adjust the aero center at high angles of attack. The airplane flew pretty nicely as it was very light and over powered (again to cope with Mexico City).

Ben