There are rules regarding the use of lights onboard an aircraft.
As already stated the rules are general. The use of them reflects your airmanship.
This is what I could call good airmanship:
The beacon is turned on before start, and turned off when the engine is completely stopped. Propeller at a standstill or turbine has winded down ( although turbines take ages, doesn't mean you should wait till n1 drops to 0%). For pistons, whenever magnetoes are on. (Rules say: Always when operating the aircraft, unless deemed conflicting to safety (FAR))
Nav light, only when they are useful, i.e when natural light is low enough for others to see the nav-lights. (Rules say: When sun is less than 6 degrees below the horizon or if light is not sufficient to see an object clearly on a distance of 3 SM (FAR))
The strobe only when entering runway. however at night or in IMC it's a good idea to turn it off as it tends to blind you. (No rules regarding the strobe (FAR))
I assume that JAR, BCAR and other Civil Aviation Regulations has similar reules to FAR. At least there is in Norway where I'm from
