My wife often puts a loose layer of aluminum foil over the meat (e.g. a chuck steak) in the slow cooker. Since there is no kind of seal, I don't think this makes any difference.
Seems to me that you might pick up a bit of metallic taste with long cooking times with acidic foods. Recipes commonly call for marinating in non-metallic containers. That said, I've done baked potatoes wrapped in foil in a slow cooker that came out just fine.