When adding to the brew wort, I add the actual nutrient for the final 10 minutes of the boil (it's on the nutrient label)... For starters, I add it at the start, since it's a short boil. I've also added it post boil without any negative effects (for starters mostly)...
In my last brew I also tossed in some cooper's yeast into the boil, to be more food for the yeast. We'll see how this brew does in the end, but it was pretty active early on. So maybe the extra food I gave it (the boiled yeast) is helping it along. It's probably the only actual good use for cooper's yeast. :rockin: