Have you ever noticed that those “Little black things on your Tires” are missing? Those are called Valve Caps. Don't they just magically seem to disappear? I bet you've noticed your missing 1 or 2. You keep driving around without replacing them, right? You most likely don't think it's a problem, and is a small issue.
Although small, valve caps help protect the valve stems from debris and dirt. If you don't have valve caps on some tires, you'll most likely notice those tires lose a little more air. Usually all the debris and dirt hit the needle, gradually releasing air. So it's good to get them replaced if you are missing some.
Valve caps are available at all auto parts stores. They are the same size on all tires, and are good to have handy. Usually they are $1 for a pack of 4. Sometimes I've seen them for $2-3, but I would check at another store. You could always poach some from someones else's tires! Of course I can't condone that! That's probably why I'm missing valve caps!