if your aim is straight SketchUp export with edges switched on, it makes sense to keep the detail reasonably low (set yourself a limit of e.g. 5cm and never draw anything smaller than that, if possible). and then, as the others said, keep the same level of detail throughout the whole model.
if you go for rendered images, as Gaieus said, the more detail the better, because you won't have any lines and therefore you won't have the problem of too many lines creating one dark mess in the distance.
silver_shadow just mentioned his 2 million polygons model. I guess he referred to his 'eye candy 2' model. thats a perfect example, how high detail is beneficial for rendering π