Having lived in Pittsburgh myself, I clayed my daily driver about once a year, as well. It seemed like a acceptable interval in that environment. I also don't do the baggie test, instead just rubbing my hand over the surface of clean paint and feeling for inconsistencies. As a reality check, I've also clayed a small portion of a flat panel, such as a hood or trunk, and then felt the difference to an unclayed area. If it feels any smoother to the touch, that's a good enough reason (for me) to clay the car.