I actually wash bagged veggies for a different reason (perhaps they're related).
I find that bagged veggies have a certain taste compared to fresh. Washing it seems to rid everything of said taste. So the habit stuck.
My lazy ass sisters don't washed pre-packaged salad mix (some packages say washed, some say "wash before consuming" but they don't give a crap). They haven't had any serious bouts with food poisoning so I'd imagine that in most cases, you'd be fine. If you get sick, there's probably a recall waiting to happen, or you just ate a bad batch.