As a matter of practice, I replace my thermostat when I change the coolant. The part is about $15 from Fastlane, and you have to drain out some of the coolant to replace the thermostat anyway. Once the coolant is out, all you do is loosen three bolts or so, take the old one out and put the new one in. So, I see it as cheap insurance.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record