I would advise against modelling holes into the bodywork.
Best example here with this car model are the lights. Their outer shape is perfectly aligned with the curvature of the body so it's going to be really difficult to make them look right if you model them afterwards. Much easier, faster and accurate if you model them with the body and then cut them off from the mesh when you want model the rest of the lights. Here's a perfect example about what I mean:
http://www.onnovanbraam.com/index.php?tutorials/polygon_modeling_7_car_body/
It's not as important with the windows and the holes in the bumpers but it's good practice on them as well. It makes sure every (parallel or continuous) surface gets modelled right.