Yes, you can do a feeder at first light but that sometimes hurts rather than helps. That is because hogs will frequently go to a feeder a half hour or so before they know it is going to go off. If you set it to go off at first light, you would need to be in the stand well before then.
Mike brings up a good point. Once you fire the first shot, the hogs will usually not come back to the feeder that day - sometimes it takes a few days for them to come back.