I think if you want to stack silver, stick to the fine silver coins. 9999 if possible, but 999 will do as well. The old small change coins are not usable for many purposes, and the more other stuff (that has not much value) the more weight and space it takes to stack x-amount of silver. So unless you can get them really cheap, or they have a collectors value, I don't see much sense in hoarding them.