I teach the blindspot check EVERY time from stationary. To be clear, the only way to do this check properly and effectively is to slightly lean forward and then turn your head round and look behind. Obviously there isn’t a totally clear view, but importantly it covers the blind spot that the mirror doesn’t see.
There are certainly times when it’s not necessary to check blind spot, but it is ALWAYS required moving away from the side of the road. I teach to do it every time from stationary as there are other occasions when it is extremely useful. So, rather than confuse anyone by explaining exactly when it is and isn’t necessary, I take the simple option and go for it every time the wheels have stopped moving. It doesn’t get much more simple than that.
Pete 