To relieve tension, I take it a step further and simply remove the chain altogether after each ride. Then I disengage the brake cables, flip the QR skewers, deflate the tires, and loosen each spoke.
Seriously though, being on the receiving end of a quality BJ relieves much more tension than any of methods listed above.
You can just have a room where you pump in some inert gas (maybe just nitrogen) after you've stored your bike and that will reduce the oxygen concentration to very low levels after a while.
The reason you leave your bike in a 39x12, is to reduce the "stretch" on the derailleur springs.
On bikes that have been sitting for years in a 53X 26, for example, the rear derailleur spring does not want to pull it down to the next gear....14 to 13 for example.
After every ride, I remove my derailleurs and put them on an aerogel support in a vacuum chamber, and I orient them so that I can minimize undo gravitational force on the springs in the derailleurs. I then pump the chamber down to 10^(-9) torr. While in the chamber, I monitor changes in spring surfaces with an STM and use xray crystalography to monitor for break down of the crystalline structure in the springs.
I read an interview with Cofidis rider Maxime Monfort who said that he always leaves his bike in 39x25 when not using it because this gear produces the least tension on the rear derailleur.
But in my LBS the other day I noticed that all the road bikes were in 39x12 and the guy told me he always leaves bikes in this extreme gear ratio because it creates the least tension on the cables?!?!
Steel springs and cables do not stretch when idle and within their operating limits. You do not need to select any particular gear to save springs or cables. This is a fallacy that has been handed down over decades, as some of our more facetious colleagues have been hinting all thread.