I found 175 is my strongly preferred to the point that my vintage bikes that have 170 on them get ridden much less. I am 6' 2" and have been to a bike fitter that uses Retul and confirmed that 175 is best. Of course smaller does not hurt, it just feels so much less power / leverage when pushing and especially climbing.

My challenge has been when I get a vintage bike I really like the search for 175 cranks has been costly to replace the 170s, especially on French cranks. I have mixed feelings but have used Velo Orange on one bike, but it is not a bike that I am concerned being a purist.