Obviously not every single OLED panel can be tested for this if the manufacturers don't do it themselves, but a few places tested OLED/AMOLED phones and found slate grey is close enough to full black in power savings. Since then I just choose the most visually pleasing theme as some full black themes are really poorly designed.

Fair enough, a choice between both options is always better.

Exactly, and because the rest is off you'll notice it earlier. It still depends on how long those pixels are on though. The longer they're on the more they degrade.

If the whole display is on all of the pixels would degrade eventually, but you'll notice it less because they all degrade.