One of the pivotal points in a model is when I start to render it. Rendering a model (using the right shadowing and occlusion) will let you know what parts need more detail and what parts should be ignored (in terms of adding more detail).

In the images below you can see that certain areas are starting to be filled in with shadows and don’t really require additional details. At this point, it’s safe to put rough representations of the components within instead of actually modeling them: see my Ducati 999 model for an idea of just how rough these can be under some circumstances.