Jean,
I have a homograft and think that it is important to have a medic alert card and bracelet/necklace. The reason is that, should you ever be in an accident or be unconscious, unable to respond, when a medic sees the scar on your chest assumptions may be made in a hurry. Since homografts are in the minority for valve replacements, certain procedures may be avoided for you because they believe you ARE on Coumadin. Also, you have some metal in your chest where your sternum was repaired that, under an MRI, are going to heat up. Finally, any and all meds you are on are important to know about in an emergency.
I have a card, a necklace, and the relevant info engraved on the switchblade I carry in my motorcyle boot (OK, I'm kidding about that last one), and even though next month I will likely not be taking any meds at all any more, I think it is important for emergency personnel to know exactly what they are--or are not--dealing with.
--John