if you're not going to dump those cois, don't even bother, because the only price that matter it's the one that you dump your coins to
as long as they remain in bitcoin form they are untoucheable if not used for purchase, and in that case the value that matter is the one at which you use them to buy
Good luck convincing the IRS of that..
Stocks are also taxed on their current worth, not their worth when you bought them. The same goes for bitcoin, gold, and whatever other assets you hold.
They might not know about your bitcoins, but that doesn't mean you don't have to pay tax over their worth.