It really depends on if you're just steeping or actually doing a partial mash, I'd think. First, though, let me say that even if pH is a big factor in tannin extraction, I still think it's possible that going _too_ hot (whatever that is!) can be a problem. But if you're using diastatic grains (those containing enzymes) for a partial mash, going over 170 will denature the enzymes and you won't get conversion. That's less important if you're steeping non diastatic grains like crystal. In that case, I'd take a "better safe than sorry" approach. That said, back in the 80s, some homebrew authors had you leave the grains in for the whole boil. In that case, like in a decoction, you probably won't get any tannins of the pH remains under about 6.0.