170 is supposed to be the edge temperature at which you can extract tannin from your grain husks
I've seen several controlled experiments that state that tannin extraction starts at 180F (mash temp, NOT wort temp), and takes a good 8-10 minutes at that temp or above to really show up as noticible in your final beer.
170 is a standard mashout temp. I've done almost 25 BIAB mashouts at 170F without any tannin mouthfeel in my final brews. I even accidentally heated my mashout wort to 212 boiling once for a good 2 minutes, but due to the thermal energy of the mash, that only heated the INTERNAL temp of the mash to about 178. Again, no tannin mouthfeel in my final brew.
Point being, my experience and most of what I've read says pay attention and keep your mash's internal temp under 180F (stir like crazy during mashout for BIAB), and you'll be fine.
It's difficult to extract tannins, you really have to work at it!