I'd do the cornes of the lowest point of the housing and a couple in the center. Won't affect the light much and water will drain out as it sloshes around.
One of my headlights used to get water/fog in it. All I did was take the headlight off, take the bulb out, and place a hairdryer so it blew hot air inside the housing until it was all dry. Then I'd check for any cracks or places where water was getting in and put it all back together. Did this about 6 times until I must have finally sealed up the spot that was leaking water in. I haven't had a problem in 3 months now.