I'm nearly 5' 11" with a 32+ inseam. Just switched from 175 to 170. (175 was what came on my used road bike when I got it.) The 170s are definitely what I need. The 175s were contributing to some minor knee pain. And it is harder to spin with longer crankarms.
If one thinks about it, the longer the crankarm, the bigger the pedaling circle. The bigger the pedaling circle, the more possible stress on knees. And the harder it is to pedal at a high cadence.
My LBS guy is 6' 2" or so. He uses 172.5 crankarms. He used to race back in the day, so he has lots of experience. He's tried everything up to 180, and prefers 172.5. He says it's all about pedaling at a cadence of 100-120. Too long a crankarm tends to prevent this.