Re: Dojo etiquette (explanation) ?
When I lived in Tokyo, I was training at my dojo one day and Koyama sensei broke training to impart some information with a clap and "sono mama" - stay were you are, i.e. no need for everyone to run back to the side. I dropped into seiza where I was, which happened to be behind sensei. Not with my back to the shomen as I was at the edge of the mat, but I was between the shomen and sensei in terms of my position along the side. A fifth dan on the other side of the mat had a few things to say to me about it......... needless to say I haven't done it since.
I've been in classes in Japan where everyones dropped into seiza in a circle around sensei whilst he teaches, which includes having their back to the shomen.The rule does vary in different places, particularly in the west, but also in Japan.
I would suggest that not being between sensei and the shomen when he teaches, or not having your back to Kaiso when being instructed by sensei is probably good etiquette and a safe way of not getting chewed out if that is the rule in that particular dojo.