A little dihedral is a good thing. The Tower Trainer .40 MkII ARF calls for 5 1/2" or so of dihedral (if one wing tip is flat against a table, the opposite side should sit 5 1/2" high from the same surface) if built per the instructions. Sanding out a bit of dihedral angle from the wing brace makes it easy to lessen the dihedral. I built mine with about 4" of dihedral and it flew very well, tracking nicely through loops and easily flying inverted.

If you build one with no dihedral, it will be as much sport plane as trainer. I think working for a 2" to 4" dihedral range will give you the best of both worlds - a sporty plane that flies aerobatics well but still can correct itself if you let off the sticks for a bit.

Most of the classic low wing sport planes still have dihedral built into the wings, it's just a matter of having the right amount of dihedral for how you want to fly.