AAV's are legal here in oregon, but like he said codes vary, and depending on the inspector, he might not allow them anyway (ours doesn't). Most things like that come in where structural conditions preclude the installation of a standard system.... and 99.9% of the time if you are a good plumber you will find a way to do it.
Anyway, as far as placement, you want them as high up as you can possibly get them, preferrably ABOVE the flood level rim of the fixture they're serving. That way if it gets stuck open, and you get a backup, it will come out of the fixture before it comes out of the AAV, which you shouldn't have to worry about.
Personally I don't like them, simply because code on venting requires airflow throughout the entire system. AAV's simply do not provide this.