Where to put one's best power cords??

Some or most power cords need in fact more than 300 hours to reach their full potential, but yeah 300 hours should get you most of the way.
Shunyata power cords are widely considered among the best overall. I thought about getting one but then the Dominus came up. I could use couple of better cords, though, on the phono and PS Audio regenerator. But this would be expensive and not so easy, Custom Power Cord Top Guns that I have are quite good, so it would be at least $500-$600 each used, thinking either Purist or Shunyata.

Jeff - the 4th response from Elizabeth was the right one. Experiment and confirm where the aftermarket power cord does the most good. While there are general rules that are true, there's no substitute for experimenting and confirming where it's used to get the best improvement.

Go to Steve Reeve’s website (image99.net), click audio alchemy, and build his DIY power cables.  I built four cables for under $450 (including silver plated connectors and Furutech signal wire) and connected everything.  Tremendous improvement.

All good wishes

1. Wherever it sounds best.
2. Wherever it looks best.
3. Wherever it fits.

I leave mine at the manufacturer. It saves the most money.