It depends on what you are trying to do.  For example, if you want to handle the ParentFormLoading event and perform the same function of code every time for that business object, then you would want to place your handler code in the BO source file.  However, if this is a one time thing, then you would want to add your handle and code on the form, form example.  So it really depends on what you are trying to accomplish.

Due to the nature of events, we are actually planning on having a section in our May training class dedicated to the understanding of how events work and how to interact with events in an application development environment.  Events, as you know, are a great tool but many times are misunderstood and not used to the best advantage of the developer many times.  So we have added this to our schedule so that some of these questions can be answered.