If you do this, just a reminder that a player might accidentily have the tactics on berserk( or do you mean as in naked or wearing armour of rage when you say true berserk ) and use the extended move command, 'w[1,2,3,4,6,7,8,9]', and run into and 'unintentially' attack a non-hostile.
Otherwise, I think coward is perfect for auto-switching and will get people into the habit of setting tactics to coward, which is good.
Edit - Wait a minute, that won't work. I don't always use the 'l'ook command to check whether a NPC is hostile. I just set tactics to berserk sometimes and attack.
So, I think the best might be if the player is always queried, even in true berserk, except in coward it auto-switches.