MTB, you need to consider both pedaling efficiency and rock/root strike clearance - with rock strike clearance likely being far more important. I moved from 175mm to 170mm and it was no big deal because my MTB fit was so different than my road fit. I was having issues with my shoe barely clipping hidden stuff and causing crashes.
Keep in mind that they likely just move the drilling on a 170 - 175 arm rather than the whole metal casting being different. When you get into the 160s you might get a different length arm.
I’d say a set of used crankarms are cheap and only take 10mins to swap out if you have all the tools.