Power cord length?

Get the exact length you need that will give the shortest distance to your outlet that won't interfere with your signal cables (and won't look like a temporary install - i.e. don't make it too short). If you have to cross over your signal cables, try do it at right angles or cut a piece of pipe insulation and put it around the power cable to maintain a gap between them. Done!

make sure the length of the cord is at least the distance between the outlet and the component

After that, I don't think it matters

I owned the most expensive MIT powercables. These days I know they are not the best anymore. The make instruments and voices too big in proportion. And yes longer powercables often sound better than shorter ones.

5ft is the optimum length.