Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of refiner Valero Energy (NYSE:VLO) fell as much as 10% today on speculation that the company's profits will be pinched by U.S. oil exports.

So what: The Commerce Department said it would allow two companies to export oil from the U.S., something that has been banned for 40 years. Companies would export oil if they can get a higher price abroad than they do by sending it to U.S. refineries; this would raise prices and lower margins for refiners.  

Now what: The U.S. energy industry has been turned upside down by the explosion of shale oil production, and this is one of the side effects. Net oil imports have been just 30.3% of consumption in the U.S. this year, down from 35.5% in the same time frame a year ago and from 60.3% in 2005. Falling imports and increasing production allow for the availability of exports, which is good for the country over the long term. The downside is that it's bad for refiners, and that's why many of their stocks are down today.