I don't know if it makes much of a difference at all. I do it both ways, depending on what time I get the whole brewday started. It also depends on how much prep work I have done before getting the pot on the stove too. It is an interesting question, and I'd like to hear a definitive answer, but I doubt it will convince me to only ever do it one way, over the other....
Also, as I recall, the instructions from 2 separate kits I have bought each said one way. A Brewers Best kit said to bring water to temp then steep, while a Northern Brewer kit said to steep while bringing water to temperature. Granted, I could be wrong in remembering, but it sure rings a bell.