Pressing cmd with the thumb feels ergonomically perfect for me on the mac-keyboard. Makes it a lot easier to include alt in shortcuts that use both cmd and alt too (there are a few).
If you never get used to it (or like me get confused by having to switch between Mac and Win) you can always remap the keys. I had to do so on my work pc as I kept my Mac habits. Keyboard shortcuts become second nature, but having two “second natures” just doesn’t work;)
@NURB I have actually set the right alt-key to show all the windows for active application. Works perfectly and I only use the left alt-key for alt anyway 