I recommend that crank length should be between 20 to 21.5% of inseam. Its not a hard and fast rule and there is a range of lengths that suits most people. Doing a quick and rough calculation, guessing an actual inseam of 31" then I calculate that 170mm cranks are at the top of your ideal range. I think you would be better staying with 170mm. Would you feel a huge difference from an extra 2.5mm?

On the shorter rides I doubt that it would make much difference but on the longer rides I suspect that it would start effecting your comfort.

Anthony