The only possible danger I can see of engaging the autopilot too early is if it has a malfuntion (not detected in ground testing) and pitches down or some other unpleasant maneuvre. To have some hundreds of feet height would give the pilot a chance to disconnect it again and gain control.
Lear 31A - 200' minimum on approach (CAT I aircraft), 1000' enroute. The latter must mean not to engange until 1000' AGL I suppose.
TD