For me personally, it is quite simple. I do what I can and when I am pleased I stop. For example, I like the idea of hoses and wires under the hood of a model, but not if it makes it look too busy or congested, make sense? Another thing I have noticed over the past couple years, everything I have started that was to have lots and lots of detail, plug wires, hoses, electrial connections, etc, are still sitting on the bench or in boxes no yet completed. However, the "quick" or "shelf" builds have been done and on display. I do one show a year, NNL East, and it is peoples choice, so if I do decide to show something, I dont worry about judges. All the other times, friends, family see what I have built. My greatest reward, has been as of late, building with my four year old, and hearing him tell me how cool it is to build a model with dad.
Long and short of it, build to you are happy...