Sales receipts are good, but if you purchased your gear at a low cost below list that is what you will receive back, assuming you have full replacement cost on your insurance coverage. If you have older models that are no longer in production, it is best to have proof of what the original list prices were for those items. You can get them from old magazine reviews etc. As long as it is in print somewhere that should be good.
I have a separate fire proof box for old magazine reviews and anything that shows what I have and what it originally cost. If they are newer items then I have the person I bought them from write them up to reflect the list price, just for insurance reasons. Along with those are photos of everything.
My policy has full replacement cost on these items and I do not need a special rider attachment. However, I have scheduled other valuable items that I own, such as several antique weapons. Scheduling things ahead of time removemes much of the hassle if you do suffer a loss and doesn't cost anything to have done. So, I am considering scheduling my major audio components. I feel it would be easier to go through this trouble now, then dealing with insurance questions later.