Check with State Farm. Here in FL they will provide coverage if you own a Doberman, and it isn't all that much more (compared to what others would try to slap you with).
As far as getting out and biting someone, a well trained, well socialized, healthy Dobe won't just go and bite someone. Ok, people get a little concerned when they see a 70-90 lb dog (regardless of breed) running towards them, but again a well socialized Dobe won't just bite. If a Dobe gets out, it's usually due to boredom, or if it's an unaltered male, there's probably a female is heat somewhere near.
We had a guy that works with the power company come through our back gate, uninvited and unannounced, and my female got him on the back of the leg. He called the cops and animal control. When I pointed out that our meter is on a pole OUTSIDE of my privacy fence, the man was charged with trespassing.
Do not use "Beware of Dog", "Guard Dog on Duty" or any signs like that, however, you can display "Doberman(s) At Play"
If you find a different Ins. Co. that charges a little more, aim for obedience and work towards the dogs Canine Good Citizenship and they might give you a bit of a break.