depends... really, it depends on the model. for LPM, it's usually a single object. for high poly, well... for exemple, if you take Ryx's orion as an exemple, I wouldn't have extruded all those bits I think, I would have made them separate objects ( much less polys ) for exemple. For the herc2, it's a few objects stuck together ( nose and central part, fin, left engine pod, right engine pod, cockpit and cockpit glass I think, plus a few details ). The mesh won't be as pretty when you look at the wireframe, but my comp is not up to ubbercomplicated meshes, so when I can use separate objects, I do ( if you want an object that looks like a big cube next to a small one, if you extrude it, you'll end up with 28 faces. if you put two separate cubes next to each other -intersecting, even -, you'll have 22 faces -delete the polys of the smaller cube that are inside the big cube. doesn't seem much, like that, but I guess if Ryx didn't extrude all those things, he would have saved a few hundreds polys at least on the orion ).
that aside, some meshes will look better made with one object ( a cup of coffee with the hadle ) and some will look much better ( or the same, but with less useless polys ) if built with multiple objects ( a radar, maybe ).
voila, I don't always follow that to the letter, really, it's all a matter of what yçou're doing, sometimes you just feel like doing that mesh this way rather than that way will be better, and that's about the best resume I could give.
