Lots of different theorys out there . . Pattern planes have had razor sharp TE's through to the current trend of thick TE's . . and flutter hasn't been a concern with any of them. Sealing the hinge gaps, correct linkage geometry and servo choice, is considered the appropriate method of handling flutter. 3D throws are a 'flutter' concern, particularly when using analog servos (not recommended), as small amounts of servo travel are used to control large surfaces for general flight (i.e. low servo resolution).

The attached pic is of Gerhard Mayr's own design . . it finished 5th at the World Champs, so I guess thick TE's work as intended for him. Definitely most new F3A designs do not have thin TE's and the rudders are surprisingly thick.

I would just build it as it is, and wouldn't bother sanding the TEs down . . doubt you would notice any difference in flight.

Cheers, JB