Well, AIRWAY, speaking for my lot, it is permitted but not encouraged. The reason for this is that we would not want a blanket prohibition in case attending/observing a colleague's sim session may do someone some good. Equally, we wouldn't want the inhibiting effect that "open house" might have on the crew doing their checks.
This way, if someone wanted to observe a sim session, he would have to have a good reason and get the agreement of the crew concerned.