Ditto what Wynne-R says... the most important factor is pH since polyphenols are more readily extracted at alkaline pHs. My experience says that they not only taste negative when the beer is fresh, but also, they do not age well, especially with oxygen exposure. Also, consider the pH of the mash you are sparging. Specifically, as the last Hansel drops so does the buffering capacity and if the sparge water is a high pH, more polyphenol extraction will take place.