What is the Optimal Tightness for Learning to Ride with SPD Pedals?
My preference is to start with the pedals wound right back so your feet fall out if you even think about unclipping. But from my experience of novices most people will fall off at least once due to being clipped in. Regardless of how loose the pedals are.

After a week or so, or when the learner starts complaining that they're unclipping unexpectedly, I crank the tension up to normal levels. I'm not a fan of any kind of gradual change. I do err a little on the loose side while they work out what tension they prefer, but not in a smooth change fashion. It's a step to "firmly fixed" in one hit.

I suspect it's more dangerous to have the tension too tight - at some point anyone will be unable to unclip without damaging themselves - your ankles are not particularly good with twisting forces. 

What I did find when I was learning was that there are time when I twist my feet in the pedals without realising it, so I was unclipping unexpectedly not because of pedalling force per se, but through half unclipping then pedalling. If you're off the saddle and throwing the bike around a bit, you may well have been pulling sideways out of the pedal. Don't do that :)