Everyone who drives a car has suffered the bad driving habits of others. When another driver cuts you off, what do you usually do? Most people will look at the other car for some information about who the driver is. If that person has a bumper sticker or decals in the windows, those become badges of what the other driver represents. That is likely how you refer to that person -- "That Breast Cancer Awareness guy just cut me off!"
The vast majority of people will not use other drivers' bumper stickers to track them down and cause them harm. For those who work in conflict-rich professions such as the law or law enforcement, however, this is not always true. If someone is seeking information about you, and only knows where you work, that person might be able to learn about you and your family from the stickers on your car. A "My kid is on the honor roll" sticker can reveal where your children go to school. This, along with a "stick figure family" in your back window can provide enough information to someone to convince your child that he or she is a family friend. Be careful with the information you broadcast to others on your vehicle.