If you change cranks from 175mm to 170mm you have wasted your time and money. I am not for a minute saying that a cyclist should always be on 175mm cranks, but I am saying that the maxim: "go big, or go home" applies here. If 175mm is too long, so is 170mm. I wouldn't go longer than 160mm and I might even look for 155mm if I felt that strongly that my cranks were too long. That is the kind of change that makes a real difference.

The human interface with the bicycle drivetrain is complicated immensely by the contribution of ankle flexion and also the length of the plantar system. Ankle flexion easily absorbs a 5mm change in crank length. People have leg length discrepancies of more than that and cycle for years without even knowing it.

If you didn't know what length cranks were on a bicycle and you needed to answer the question: "what should you do, raise or lower the saddle?", I doubt anyone could reliably answer the question with a 5mm change. Make it a 20mm change, and I doubt that anyone could not reliably answer the same question.

If changing cranks was as easy as changing saddles, this discussion might be entirely moot. But changing cranks is a big deal. It can also be very expensive, especially when a significant percent of cyclists would need a bike repair professional to do the work. So, make it count is all I'm saying.