How many cases of collapsed trachea in dogs are avoidable?

One of the only main health issues I seem to need to worry about with my new puppy seem to be collapsed trachea. She's part pom and they're apparently infamous for it. Some of the information on the web is a bit confusing-some sites imply that it is inherited and unavoidable, while some sites say that it is caused by using a collar instead of a harness or not being careful enough to avoid neck trauma to the dog. How common are problems in a 7-9 lb pomeranian/chihuahua mix when the owner has taken all precautions? For example, she'll never, ever, even once be walked with a collar or even a harness that puts any pressure on the neck. I'm also so careful about trauma with my animals that my elderly blind ferret has never even gotten hurt and I play with her all the time. I am also going to work with a canine nutritionist to provide the most perfect diet possible.