Wag kayo mag rear fog light kung walang fog or if it's not raining HARD. It doesn't matter if you're driving on NLEX/SLEX... kung walang fog or NAPAKALAKAS na ulan, keep your rear fog lamps OFF. Utang na loob.
Fog attenuates/absorbs light and so the standard tail lights tend to disappear in foggy environment. Hence, in such a situation, you should turn on the rear fog lamp(s). Otherwise, keep them OFF.