Having just gone through the research, I can say with confidence to change your tires every 4-5 years, regardless of their physical appearance - earlier if they're underinflated or if you two at speeds faster than 65mph. The tire material degrades naturally over time, and faster when "abused".
It's been an exensive week for me - Tuned up the engine, replaced 4 deep-cycle batteries, and replaced 5 tires on the trailer.
More info:
http://www.discounttire.com/dtcs/infoSeparationAnxiety.dos
http://www.discounttire.com/dtcs/infoTrailerTireFacts.dos
BTW - I'm in no way endorsing Discount Tire Co. In fact, I would never do business with them, but that's my personal choice. That said, the info in the links are valid.