I've used Neem Oil forever. Use it only when you detect a pest/mold problem. It is not a cure-all, but does eradicate many of the pests, mold/fungi, and diseases that you may encounter in your grow room. You just don't spray and forget it either because the effects dissipate quickly. Neem should be applied in 2-3 day cycles until the problem ceases. And when you apply it, turn off all HID's for about 15-30 minutes to prevent leaf burn. Turn your fan/s off when spraying Neem. Then turn them on again after you spray. I've used Neem during flowering when I've had too, and would recommend using it up until the last week or so of harvest to avoid any residual smell or taste. I always keep a bottle of the stuff around.
P.S. mix only the amount you are going to use because as I said before, Neem loses it's potency after extended periods of time. And add a couple of drops of dish washing soap to break up the oil and you should be good.