Whether to caulk around a light fixture depends mostly on the kind of light fixture. In some cases, the caulk can provide a seal against colder or hotter air in the ceiling. In others, the caulk is just a cosmetic measure. In still others, caulk isn't necessary. Consider the following when deciding whether to caulk around a light fixture in your home.
Insulation
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The primary purpose of caulk is to help seal a seam, hole or crack against outside weather. This is why builders caulk around windows and doors. If your ceiling or wall is insulated, it's rarely necessary to caulk your light fixture against cold air. The same goes for interior walls. If your roof doesn't leak, you don't need to caulk your light fixture against rain or other precipitation.
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If your ceiling or wall isn't insulated, or if you feel a draft despite the insulation, consider running a bead of caulk around the edge of your light fixture.
Kind of Light Fixture
The kind of light fixture you install will have a lot to do with whether you want to caulk. There are essentially two kinds of light fixtures: those with a visible base and those without a visible base. All light fixtures have a base where the fixture comes into contact with the wall or ceiling. In some fixtures, that base is plainly visible. In others, it's obscured behind a larger fixture for the light bulbs themselves, or by decoration.
If the base of your light fixture isn't visible, there's no need caulk for cosmetic reasons. Also, it can be very difficult to caulk a fixture under those circumstances. If the base of the fixture is visible, you may or may not want to caulk for cosmetic reasons.
Appearance
If the base of your light fixture is visible, check around the base for gaps in the drywall. When a light fixture is installed, the builder has to knock a hole in the drywall sheet for the wiring to come through. In most cases, the hole will be smaller than the base of the light fixture. If it's larger, the result is an ugly gap. Because the hole needed for wiring is usually very small, this sort of thing usually happens when replacing a light fixture with a new fixture that has a smaller base.
If there is a gap near your base, you can often fix it with a bead of caulk. Even if the gap appears in only one place, run the bead all the way around the base. This looks much better than just a patch of caulk at one point around the base.