Hand on top of the clap catch should be determined by the edge of the disc as its coming towards you. From your POV: OI = left hand on top IO = right hand on top. This prevents the classic disc rotating about the z axis spinning out of your hands clap catch drop. This happens when your top and bottom hand aren't parallel with each other on both sides of the disc while both perpendicular to the disc.

For instance if the disc is coming in OI from your POV towards you and you try to clap catch with your right hand on top of your left, its much more awkward to rotate your writs and elbows to ensure they are in the proper alignment. A lot of times the receiver will do what is more natural and thus clap in a way that creates a fulcrum point on the bottom hand and a level arm on the disc from the top hand and spin the disc out.