I say open it. If you haven't had it yet, it's a free dram of 18 year old whisky. Since its a relatively mass produced product (both the 12 and 18), I would question it's collector value in the future. Could I be wrong? Absolutely.
At my local shop, you can get a mini of HP with purchase of a bottle of famous grouse. I don't see that being in any auctions in the future.
PS, if you have had the pleasure of having HP 18, I know a guy who would love to take it off your hands
Our local shops don't have any promotions with HP 12... Oh well