In my humble opinion, whether to add fat at all depends on how you plan to cook the ground venison. If used in chili, meatloaf, spaghetti, shepherd's pie, etc., I think it is best not to add any fat at all. If it is going to be grilled as burgers, then fat helps it stay together and not dry out. Since I use nearly all of my ground venison in the former way, I don't add any fat, so I can't answer the question about which type fat, but I do think that that the aforementioned uses benefit greatly from lean ground venison as opposed to ground beef with its inherent fat.
Nice problem to have, congrats on your elk.