That's probably what I was taught, but in practice why? What's my left hand going to do with all its hundreds of milliseconds of free time while my right types a capital letter?
Eh? If I hit shift with my left and the letter with my right, that can't possibly be more stretching (it's probably less) than hitting both (the other) shift and the letter with my right?
For me it's more a matter of putting as little stress on my hands as possible. Pressing two buttons (shift and key) with one hand is just a little less comfortable than pressing just one key.