I've used epoxy OK, but I used a custom mix of boat building epoxy and special filler which makes for a more flexible cured resin than the stuff that comes in tubes. If you put the post in with the epoxy uncured I suggest you block up the end of the post with melted candlewax so that epoxy cannot possibly get on the threads. If you want to fill the hole and redrill - if one of the holes was slightly mislocated for instance - then on the whole, as Simon says, I suggest most folks will be better off filling it with wood and using a wood glue rather than epoxy. The one thing to avoid is trying to push the post into epoxy that's fully cured and hard.