I think Triumph (and many other manufacturers) state that bolts, washers on drain plugs, gaskets and numerous other items should be replaced after a single use because of liability issues. They don't know who will be working on their bikes.
I personally (to the best of my knowledge) have never replaced a copper/aluminum washer on a drain plug or a bolt on a brake rotor and have tried to reuse things like valve cover gaskets if they looked fine often because the parts aren't readily available. Experience plays a big part in this. Tire manufacturers state car tires are for cars, yet fellows place them on their motorcycles... Is that life threatening? Placing a rear motorcycle tire on the front forks with the tread reversed is supposed to be a no no.... but fellows do it? Our engines are designed to run with certain compression pistons along with certain cam profiles. Do we change them? Do we modify the engine tunes? Is there a risk the engine could blow or seize causing a life threatening injury? What about those guys that reduce the torque on the rear wheel axle to a lower value? I guess it depends on your risk tolerance. If you do not have any experience in the field, following the manufacturers recommendation is probably the best thing and the most expensive.
Honestly, how many fellows have had their brake rotor come loose due to reusing bolts??? I never have.
