I consider the use of mixing in a 3D plane to be somewhat akin to those horrible "snap roll buttons" that used to litter the escutcheons of transmitters in the 1980s -- they're really not needed and more of a gimick.

If you've got coupling in knife-edge flight then learn how to trim your plane better (reflex or camber the ailerons to reduce the effect -- experiment with your CG) or, if all that fails, just get a better plane or learn to fly it on the sticks.

Sure, some birds can be a real dog in KE and require a snot-load of elevator to keep them heading straight -- but that's why 3D is so challenging and so much fun -- because it's not always easy.

I've seen too many people spend more time fiddling with mixes, rates and other stuff than actually flying. If they'd spent that time in the air practicing then they'd actually be flying better (and with less hassle) than they are now.

Mixes are a crutch -- learn to walk without them and you'll be a much better pilot.

All IMHO of course :-)