When I think of the number of polls on here, this thread might have merited one, but having said that, I'm lost as to who would be interested in the result.
FWIW, I've always mounted from the left, which seems a no-brainer if you ride on the left, but I wouldn't be prescriptive. Dismounting on the left naturally follows, but again, I wouldn't be prescriptive. Putting a foot down eg in traffic is neither one nor the other but it seems instinctive to use the same foot. I'm right-handed so I've no idea if that affects it, but I do prefer to push off with my right leg. The only time this has had any effect was a little bit when riding fixed-wheel, when you can't simply lift your preferred foot ready for the off, but a combination of track-standing and occasional lifting the back wheel to be able to get the preferred foot up works for me; and with a coaster - back-pedal - brake. This was a bit of a nuisance when I first got my shopping bike, especially as I was often using fully-laden panniers. There were a couple of times when I had the "wrong" ie left foot at the top and restarting was irritating. I soon got into the habit of braking with my left foot so my right was at the top when I stopped, ready to push off.
Re the thing about training beginners to use one foot or the other, what a load of rubbish. I'd take a lot of convincing that this wasn't just another of those things that is easy for an examiner to observe, so work backwards to justify its inclusion. It's being unnecessarily prescriptive.
