True, at least I sincerely hope not! Good point; sheer habit I suppose. It stems from the days I did 'real' rallies - in the speed sense of the description.
Now you've mentioned it, when planning the event I just assumed everyone knew their blood group, so if something silly were to occur en route (even an everyday accident) it would be good information to have available if an ambulance had to be called. Thinking about it now, guess not everyone does know thier blood group.
This being the case, now you've brought up the subject don't worry about this rule. If you know it great, if not, then don't concern yourself as it's not essential. Old habits die hard!