Two points...

First, What Bruce said: taper it wrong and you're liable to flutter. Once you see a plane vibrate itself apart that way, you get real reluctant to allow it in your own plane!

Second, you mention the tailfeathers being "slightly better" -- in what way? That should depend on YOU. If you're modeling an all-metal warbird, a tapered surface would be a truer representation of the shape of the prototype. But if you're modeling a current-day full-scale aerobat (CAP, Laser, etc.) or a fabric covered light plane (Cub, Citrabria, Pober Pixie, etc.), their prototypes have flat plate control surfaces. Aerodynamically -- if you want bone-jarring precision aerobatics, then flat plate with crisp trailing edge is best, but if you want a little gentler response about neutral, then some taper is good (although the soft neutral can be simulated by many transmitters these days).

So, I'm thinking there's nothing wrong with either tapered or flat per se -- what's better depends on what you're modeling and what you're modeling it for.