Would it be a good idea to use dihedral on a biplane?
If your aircraft is meant to be a "free flight" model, meaning that it flies without any guidance from any pilot, then generous dihedral will help keep the wings level.

Even if your aircraft is meant to fly under radio control, so long as the mission is load-carrying rather than aerobatics, some dihedral will help ease the pilot workload.  You may want to mix some rudder to your aileron channel to help it turn better, otherwise the dihedral will tend to create some resistance to aileron roll/turn inputs.  If you don't have a rudder, don't use very much dihedral.  If you have only a rudder and no ailerons, use lots of dihedral.

Many full-scale (piloted) biplanes do in fact have dihedral on one or both sets of wings.  Most typically the lower wing has dihedral while the upper wing is straight, because that helps improve ground clearance of the lower wingtips, but in some cases both sets of wings have equal or nearly equal dihedral.  (See for example this 3-view diagram of the "Bristol Fighter".  The combination of a straight lower wing and a dihedral upper wing has never been used before, to the best of my knowledge-- here's your chance to be the first!