In a fun fly design, you want neutral roll stability so the plane will hold a bank and you want zero roll, and pitch, coupling with rudder.

Several things give dihedral effect. These are dihedral, vertical wing location and wing sweep.

Dihedral, sweep forward and a high wing location will cause roll in the direction of the rudder. Try an Ultra Stick with a flat, high wing location. It rolls in the direction of rudder. Not as moch as a trainer with a high wing and dihedral, but it is still flyable with rudder.

Anhedral, sweep forward and a low wing location will cause roll opposite to the rudder. I built an original, low wing plane with no dihedral back in the 70's. The thing rolled backwards to rudder. I can you no amount of rudder below the thrust line will fix the problem. I finally sawed it in two and put in some dihedral.

A mid wing location, a straight wing with zero dihedral should have no roll coupling. Most of the profile fun fly planes are this configuration and they are normally very good in knife edge.

If you are planning on a low wing plane, you will need a little dihedral to keep from having a reverse roll when you apply rudder for knife edge. What a designer has to do is make an experienced guess as to the correct dihedral, then fly and modify. This means cutting the wing apart, chenging the dihedral and glassing it back together and flying and trying again. Having done this many times, I can tell you it really hurts to saw a wing in two the first time. After you make the plane fly better, you'll think it wasn't so bad after all.

Lately, I have been making high wing planes fly knife edge correctly by using anhedral. About 3 inches on a 60 inch wing seems to work well. Here's my Kangke SK/50 shoulder wing sport plane modified for anhedral.