Are we talking about front fog lights or rear?
Rear fog lights should only be used when visibility is very very very bad. They are the same intensity as brake lights so if misused may reduce the chances of the following traffic seeing your brake lights and thus could cause an accident. Front fog lights however are aimed low to get under the fog and light up the road surface so shouldn't dazzle on coming traffic. They should be used whenever it is even remotely justifiable, i.e there is the slightest bit of mist in a field somewhere. Front fog lights look cool, fact!!!