Whenever you find yourself doing anything involved with trigonometry during the development of a 3d game, a little alarm bell should sound at the back of your mind - could you accomplish your aims more elegantly and efficiently by the application of a few vector cross and dot products? usually the answer is yes, and you should dump all those mouldy old atan2()s like a hot air balloon dropping sand bags.
Make sure your inappropriate over-reliance on trigonometry is properly recorded in the office accident book, and then move on. Don't look back - a nasty little trig fairy is lurking back there, just waiting for an opportunity to trepan your canoodle with a rusty old funnel-cap.
