There is no hardcore solid rule, only best practices I guess. For me what I tend to do is model things as in real life industrial processes would. So if it’s a motorcycle, how are they put together in real life, I follow that approach. An office chair with wheels for example, each modeled part would be separated as they are in the chair for reals. It wouldn’t make any sense to have the wheels attached to the bottom piece, the wheels would be separated, a water bottle does make sense to be one single mesh, expect for the cap, that I would make separated, and so on and so forth.
