You should always keep your car in gear when driving, because...if for some reason you need to react quickly to get out of a situation, aka, accident, your car will be in gear already and thats less time you will need to waste when reacting. As for the track, you will always want your car in the best sutied gear. I will say you should always be in gear, cornering or whenever, also when decelerating. When you drive an automatic there is always engine braking occuring when not on the gas, this should always be the case with a manual also, i'm not saying that people don't over do it but you should always have your car in gear when declerating, this will not hurt your engine, it will not use gas up anymore because your not on the throttle giving it anymore gas than when its idiling, you will also prevent premature brake wear. And this is not going to wear your clutch that much faster if, once again, you don't over do it, there is no need to downshift to 5000 rpms but keeping your car at say around 3k while stopping will help prevent alot of premature wear on your braking system and is also alot safer.