The mount for the 171 is -2.5cm from true, -3 is for the 191's.
You're honestly not going to feel a lot of difference, but in our tests last year, majority of people thought around -3 was money. You can certainly center mount them, but they ride fine switch in powder at our recommended mark, the 140mm wide tails keep you floating pretty darn good. If it were me, I'd mount mine at the mark, just because I'm only landing something switch once or twice every day, airing straight instead of spinning 5's off cliffs... yeah.
Little of what I do has any meaning or lasting significance.