are there some classes that should NOT be subclassed? (If not, why not?)

Depends on which version of VFP you are using < s >. In versions prior to 8, I would say that it is a complete waste of time to subclass pages and option buttons. This is because in earlier versions of VFP, even though there is nothing to prevent you from creating these subclasses, you cannot use them at design time, and trying to use them at runtime is a real PITA.