Beat me to it, Backwoodsman.

We've got 7 guitars, a mandolin, flute, cello and clarinet insured on our contents insurance, including travelling with them, and none have required formal valuations. In fact, most of them haven't required a separate entry on the form (there's a value cutoff for individual items).

In the last 10 years we've claimed for a stolen flute and a stolen clarinet (both nicked from our kids) and the insurers paid out on the list prices with no quibbles or questions.

As you say, a one off classic instrument of high value may be a different kettle of fish.