Everything you ever wanted to know about Irish Moss or Isinglass finings.
IIRC it doesn't matter. The heat breaks down the carrageenan so that it is soluable in the wort. IM is most effective around 30*F to 50*F. Been a while since I have read the links and could be off a bit on the numbers.
The reason I posted the poll was I put the moss right into my boil. But then I had to clean it off my IC. Not bad, but one more step. I was looking for a way not to have to scrub all 50' of my IC every time I use it. I also figured that if I put it in the hop bag it would just coagulate the hops and goo up the bag. So I didn't want to go that route, unless other peeps have tried it.