Yeah, Valium's monstrously addictive. Never dabbled in it myself, because it seems that some of the "harder" drugs are actually less addictive (except heroin and meth, obviously).
I say you're likely to get caught robbing a bank because the security systems are so high there. Plus, even if you escaped, there'd be a lot of witnesses, so you'd have to go into hiding for awhile. With a bit of aggression and the appropriate firepower, an armoured truck seems to be a more likely target.