Bring light, and plenty of it. A headlamp only weighs a couple of ounces, and spare batteries only a few more, so bring them anytime there is any chance of being stuck out after dark (even if you intend on being back before the sun goes down). There is often an argument between flashlight and headlamp users, and while both have benefits, headlamps tend to win out, especially for people who use trekking poles.
The one big advantage with a regular flashlight is that it can give better spot illumination when you are scouting trail in the dark. For example, I tend to carry both when I do tricky trails like Cactus to Clouds, as there are portions at the beginning where the trail is very easy to use.
Remember though: your headlamps and flashlights are only good so long as they have energy. Bringing extra batteries is essential if you are going to be using your lights heavily. Getting stuck and lost in the dark is no fun at all (been there, done that).