The pilots over on the UA Pilot Q&A thread suggest that pre-departure is better (especially midway through the boarding process or later), since the pilots have typically finished most of their pre-departure checks and are just sitting around waiting for the self-loading freight to finish boarding. After arrival, the pilots are anxious to get everyone off the plane as quickly as possible and get their stuff packed up because they're not being paid at that point and are just waiting for the SLF to offload so they can go home (or to the hotel).

All of the pilots on that thread univerally say they are always happy to visit with folks of any age, though.