Jehovahenker: My usual rule of thumb is if you could dismantle it in real life, leave the meshes separate. If you couldn't, join them up. For example, a car tyre could be removed from the wheel, so I would make them as two separate objects and group them. On the other hand a head cannot (or more accurately SHOULD not) be dismantled from the rest of the body, so they should ultimately be one mesh, unless there is a very specific reason to keep them separate.
My reasoning for keeping certain meshes separate is that models made for one project could be reused in a later project, but in a different way. For example, the bench I made for this project could be re-rendered in a rough setting, something like Kubajzz's (awesome) render, but to make it look right it would need to be distressed and vandalised. Removing planks, bending bolts and tweaking textures is far easier with separate meshes than if it was all joined up with a complex UV map. Conversely, the reason for joining meshes, particularly with organic structures, is to more accurately represent reality and to avoid problems with subdivision, as well as keeping the poly count down I suppose. People generally don't have a seam twixt head and neck... at least nobody I know does.
Headwax: Another triumph sir! Brilliant as usual!
