low wing with no dihedral tends to produce a panes that's laterally unstable which isn't a big issue if you are an experienced flyer but isn't really ideal. A small amount of dihedral is usually a good idea for low wing models (a couple of degrees maybe). For mid wing zero dihedral works well.
This assumes you are talking about a aileron controlled aerobatic model where you don't want any significant lateral stability ('self righting' tendency). If you want the plane to be very stable and/or fly on rudder only then you will need lots of dihedral.