Not really. Just because the engine is running cool, doesn't mean it's OK to lean it more. Overlean, is ovelean regardless of engine temps. It's kind of a less extreme version of what people see when running in the winter. In the winter, I have to fight to keep my engine temps over 200 by wrapping all kinds of stuff around my engine. Although my engine doesn't even want to hit 200, I still know it is properly tuned, by performance, sound, smoke etc... if I were to lean it out simply because it isn't hot, I would wear it out much faster than I should.

That is why most people will warn you to tune by performance, sound, smoke, temperature and feel, rather than temperature alone. Running too lean will kill an engine regardless of temperatures. Also, running an engine to cold will kill ot prematurely regardless of fuel/air mixture. Tuning/Maintaining a little high performance nitro engine is a balancing act between proper air/fuel mixture and proper temperature.

If your engine is running with a good temp and tune without a hole, then why bother. Cutting a hole, weakens the body, allows additional dirt to get in, and if your engine is running at optimal temps without the hole, then it may run too cool with the hole. I have a trx 2.5 R in my rustler that I have a hard time keeping up over 200. If I run my new CP body, which has no holes and covers up the car real good, then I can just get the temps over 200 consistently. If I put the stock rusty body back on it, which has a hole in the windshield and is cut up much higher on the sides, the damn engine wants to run around 170-190, which is a bit colder than I want it to run, so in my case, I want the body to cover as much as possible.