In general the precautionary principle is useful in tactical situations not strategic.
I disagree with the Iraq analogy though. The precautionary principle may have been part of the reasoning for the war. The part used to convince the world it was necessary. The real problem was repeated line drawing.
I'm thinking mainly from a British viewpoint in using the Iraq invasion as an example of use of the precautionary principle. In the US I've no doubt that Saddam's defiant, uncooperative behaviour over 12 years following the Gulf War was a big factor in the decision to invade. I think another big factor would be the fact that the US had to station thousands of troops in Saudi Arabia for 12 years because of the possiblity of Saddam invading Saudi Arabia. Islamic groups used the US troops to claim Saudi Arabia was a client state of the US, and the US troop presence was supposedly the main reason why virtually all the 9/11 participants were from Saudi Arabia.
But in the UK it was all about whether Saddam had WMD as to whether British troops were committed to the invasion. 412 British MPs voted in favour of invasion, 149 against. When it later became apparent that Saddam didn't have WMD the political class tried to protect itself by calling for public inquiries. There's been two of them so far and another one is on the way.