Does rebuilding a hub increase efficiency? I was recently helping a friend build a bike from scratch, and one of the first things he did was to inspect the hub to check that the wheel was rotating smoothly. He was adjusting the cone and lock nuts.

How large (or small) of an efficiency boost do you see from such adjustments? I'm a commuter using a single-speed daily -- if I were to rebuild my hub and do the same adjustments, would I see a significant improvement?

Sorry for the newbie question! Thanks in advance.
For a typical rebuild the increase in efficiency is negligible. Think about the leverage you have over the bearings on a rotating wheel- it's pretty substantial compared to the friction of the bearings, even if they're not in perfect shape. The bearings would have to be destroyed for it to make a significant difference, and if the bearings are destroyed it's likely that the hub would be too, at which point a rebuild is moot.