I've been watercooling computers for a while and found that additives don't do much. The only reason antifreeze is around is because of the extreme temperatures it is subjected to. It raises the boiling point of the water while lowering the freezing point.
That being said, unless it helps with either of those two cases (the liquid boiling or freezing), I wouldn't do it. If I were to do it, I'd probably go distilled water with just a little antifreeze (maybe 1/3 antifreeze) to keep it from freezing.
