Technically, it is only the owners and uncompensated guests. In reality, your room key will likely say DVC Member, and the valet may accept that for free valet parking, but they could also ask to see your DVC Membership card. This is true for all onsite perks and discounts.
Valet parking is the one perk that I am more often than not asked to see my Blue Membership Card for. Last trip I wasn't even asked to see it for buying DVC pins but I was for valet parking.
