On low wing models some dihedral is often used to avoid adverse rolling tendency when using a lot of rudder. The old Topflight Contender was a good case in point. Low wing with no dihedral at all. When trying to wingover or knife edge folks report that the model wants to roll away from the yaw slightly. Even a couple of degrees of dihedral in that case would have stopped that tendency.

So a 2 or 3 degrees would be a good thing. If you want the model to be self righting to some degree you'll need more for a low wing, as much as 4 or 5. If you intend on three channel control (no ailerons) or want even more self righting ability then more is needed. For this last case there should be enough that the wingtips are at least as high as the thrust line as a general rule of thumb.