I have a vanity plate, it's almost a must with a delorean. That being said, if I saw one with DONTASK I would figure the owner is a jerk/unfriendly and is trying to say how much the car is worth and to dont ask the price. Not necessarily meaning dont ask about flux capacitor, bttf etc...
Just my thoughts
The positive reactions you get with the car outweigh any negative stupid comments so it's worth it!