Depends on how much it costs. I know someone who just dropped $9K on an instrument, and they could have spent a lot more. And it is a portable instrument. I would insure that.

On the other hand, I got a clarinet on eBay for under $200, though folks at a local music store said they would sell it for over $1K. I did not insure the clarinet, though maybe I should have (because it is my eldest, who loses/destroys stuff far more often than her younger sisters).