It's worth it, trust me. The 175s were fine, but I did have quite a few more pedal strikes than in the past, and the extra leverage required, well, needless to say, I'm finally swapping out for 170s.
Yeah - the deciding data point for me was when I said, "Well, how much would it cost to swap them out later?" and he quoted some number like $350 (they're fancy pants cranks).
The data points that led up to this decision:
* We're pretty sure when I demo'd the bike, it had 170s (the bike was out yesterday, so we couldn't check), and I liked riding it
* I am very sensitive to pedal strikes (not that I expect to avoid them completely, but if I can skip some, that's better)
* Usually for people my height he prefers 170mm
The question mark was the tradeoff. My old bike has 175mm, and I never minded (but it has a higher BB). I was told that a with a shorter crank, it will be easier to grind up climbs - with a longer crank, I get more "oomph" to pedal the back wheel up a lip. I do like that oomph, but I already knew that the Yeti with shorter cranks has more oomph than my 26er with longer cranks, so it's fine.
Honestly, I'm sure I would have been 100% fine with 175, but it would be silly to take something "fine" instead of "optimal" just to get the bike a day earlier.