I like to keep the AP on for approaches to minimums simply for the sake of the missed. The 170 is a kick in the butt during a two engine go around. Its especially fast to a low level off missed. For that reason, I like to be able to just hit the TOGA button and observe the airplane go missed while I deal with all the configuration changes that are going on. It can be done by hand and should be in VMC or during sim training, but I'd rather free up the brain bytes for the other stuff going on.