I added mine to my homeowners insurance. You can get them appraised if you really want to otherwise you just state an amount you feel they are worth. They will ask for the gun make, model, serial number, and if they are in a safe or field use. You might want to take a picture of the gun plus the serial number and keep it safe somewhere. I e-mailed all my pictures to my agent.
You definitely should do this. It does not cost much and will cover your ass if you sell a gun and it is used in a crime. Sometimes a paper trail is a good thing.