For most people it won't make any real difference in efficiency. You should be able to ride what feels best to you without worrying if you are reducing power output or efficiency. 170 mm cranks feel 'right' to me I, or so I thought, but I swapped the crankset on my commuter from 170 to 175 to get a better chainline (already had the Deore crankset in my stash) and really couldn't tell the difference after an hour or so. Six months later I'm still riding it.

http://wattagetraining.com/files/JMa...gTechnique.pdf