If you do find your headlights yellowing you might try using a mixture of baking soda and water to clear up the lenses. I usually use a couple spoons full of baking soda mixed with enough water to make a runny goop. Then rub it on and buff it off. Works great, is basically free, and no harsh chemicals are used up in the process.
Been doing this for years on our older project cars and it works great. I have not had a post model year 2000 vehicle turn yellow but if I ever do I'll try this method first before I spend $25 bucks on a product that may or may not yield your desired results.