Power cord - 5 ft vs 10 ft ?

Short as comfortable.  Using shielded will ensure you have the best noise blocking (both in and out). 

There is something to be said about using a longer shielded cable.  A little R and C can help reduce noise, but by a tiny amount. 

Dear @jumia, If you’ve heard "many things" about the length of power cables, you may be reading too many audio boards! 😉 There really isn’t that much to say!

If this is an effect at all, it’s vanishingly small compared to things such as speaker and component selection, speaker and listener positioning, acoustics, subwoofer positioning and integration, choice of listening chair, and so on. And the more subtle of those are absolutely swamped by variations among recordings.

As a rule, one could do far worse than "long enough to reach the wall outlet."

as short as possible.  as long as necessary for comfortable routing.  Avoid coils and loops.  

these are rules of thumb that are based on physics.  that said. you probably won't hear a differnce between 5' and 10' if both are good quality heavy gauge (10 awg) cable.

It's not length son that's important, it's girth.

Shorter if possible. Same thing with interconnects.