score:6
Realistically, assuming you can get permission to visit Tibet (the permission for which is separate from the rest of China), the most dangerous part of a visit by a Westerner seems to be the actual flight into Lhasa Gonggar airport.
This is recognized by many pilots as one of the most dangerous landings in the world. Please note that there haven't been more accidents than 'normal,' it is just a very difficult landing which requires special training and only those pilots who are especially licensed for the airport are allowed to land at it.
Crime against tourists in Tibet is almost wholly unheard of. There are the sorts of petty crimes that occur anywhere in the world, such as pick-pocketing at the markets, but by and large, tourists are not subjected to it.
Enjoy your trip to the top of the world, and know that you've gone to a place most of us have never or could never go!