I used to be in the "doesn't matter, don't care" camp. I'm 5'11", 33" inseam, and had bikes with 170, 172.5 and 175 cranks. They all seemed okay to me.

Until a year or so ago, while recovering from being hit by a car. No impact to my knees, but the other stuff affected my overall posture and eventually filtered down to my knees.

Now the bike with 175 cranks demands careful saddle height adjustment. If it's too low I'll get knee pain from pressure at the top of each stroke; too high, and I get lower back/hip pain from rocking in the saddle. I don't feel any sense of leverage and still tend to spin rather than mash on that bike.

So I mostly stick with 170 or 172.5 cranks. The only time I noticed any difference was when I switched a set of 52/42 Biopace rings from the 170 to 172.5 cranks. Within a few rides using the slightly longer cranks I had knee twinges. My cadence felt herky jerky. Switching back to the 170 cranks solved that problem. But with conventional round rings I don't really notice any difference between 170 and 172.5.

Some day I'm gonna try some 165 cranks, if I can find the right frame project. I have a feeling it'll be easier on my knees about 10 years from now.