It's what my Opposable thumbs were designed for.
We're thumb people Neil !.
First fingers, fore fingers, index, or whatever you'd like to call them, are what rookies use.
It's a thumbs up for a button.
A nice big tactile button for my thumb, my right thumb actually (I'm not an ambi-thumb user). Using the left is just wrong in any case. Thumb/hand - Mouth/lung coordination is what it's all about and it's a fine art. Some might even say it is a skill, and i for one wouldn't disagree.
The real question should be: Do you use your index finger or thumb ?.