I've also never found the "standard" fingering to be optimal either, and instead go for "closest finger that's available", which is similar but not exactly the same as "proper typing".

Typing that way probably uses more mental energy

I'm not sure about that, because it happens almost unconsciously; I never have to consciously think about where the keys are or what fingers to use, "it just comes naturally".

Until now I've not thought about my "rest position" either, because unless I'm actually typing something, my right hand is probably on the mouse and my left resting elsewhere. But now that you mention it, when I pause in the middle of typing and look down, my fingers are sitting on the key they last hit. If I prepare my hands like they're about to type something, then my fingers are not resting on any particular keys either --- with the exception of f and j, they're resting somewhere between keys. I think it's all "muscle memory"; it's become so ingrained through practice that whereever my hands are they'll automatically find the right key when I start typing...

Related: http://www.onehandkeyboard.org/standard-qwerty-finger-placem...