The impression I have, is that they don't bother unless you are a company. I recently bought another 60's gibson bass (maho/rosewood neck) and did'nt fill out anything. There seems to be no automatic customs procedure to pick out and look at guitars/parts, like they do when you try to order a gallon of cellulose paint..
That may be normal when you're shipping and receiving, but when you're a traveling musician at customs, you could still run into a big problem.
