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...
Let's number the fingers as follows: pinky is 1, ring is 2, and so on to the thumb, which is 5. In the following, a letter followed by a number means you type that letter with the corresponding numbered finger. If there is a ' after the number, it means right hand, otherwise left hand.
In proper touch typing, the word "unsigned" is: u4' n4' s2 i3' g4 n4' e3 d3
I find this is faster and less stressful: u3' n4' s2 i3' g4 n4' (e2 d3 | e3 d4)
That was what I was probably most likely to do before I learned proper touch typing, and I'll still do it unless I'm purposefully trying to stick to "proper". I say "most likely" because my typing was not fixed. It was more that I had a mental map of the keyboard and where my hands and fingers currently were, and I'd figure out an optimum or near optimum way to type the next word or two given those initial conditions, with no constraint that I end up in any particular position. Repeat for the next couple of words starting from whatever position the prior optimum sequence ended in. So how I typed any particular word would depend on what had come before.
Typing that way probably uses more mental energy--I wasn't consciously aware of it but something in my brain had to be computing all those varying sequences, and that cannot have been free. I know that if I was pulling an all nighter my typing would get sloppier when I was really tired, and if I switched to strict "proper" touch typing I'd do better.
I still use a mix, largely because I'm a big, wide guy using an Apple keyboard. Trying to keep my rest position on home row makes me look like I'm doing a poor impersonation of a T-Rex having a stroke. My comfortable rest position is more along the lines of w1 e2 r3 f4 and 5 just below the space bar, and left actually on the "proper" home keys. (The asymmetry is because my second monitor is on the left, so I tend to be turned toward the left).
My recommendation: learn proper touch typing. It gives you something that won't use extra mental energy and works reasonably well on most keyboards. It gives a base you can always stand on.
Don't force yourself to always use it on all keyboards, though, if you find something else works better for you, especially on the keyboard you use the most.