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

The Dow Jones Industrial Average (DJINDICES:^DJI) opened on a negative note this morning, falling 55 points as of 10:45 a.m. EST and threatening to post its third straight day of declines. Without an obvious cause, most traders blamed the declines on the lingering fears of what the Federal Reserve could do to hurt the stock market's impressive bull run this year. In the absence of major macroeconomic news, stocks fell largely due to individual-company issues, and Disney (NYSE:DIS), DuPont (NYSE:DD), and UnitedHealth Group (NYSE:UNH) are all among today's decliners.

Disney has dropped 1.5% after analysts at B. Riley downgraded the multimedia giant. Yet despite the move, investors should note that the reason given for the downgrade was valuation, which reflects Disney's impressive recent share-price gains. With the market at record highs, valuation-based downgrades have gotten a lot more common, but they don't necessarily speak to any change in the prospects for the company itself. Disney shares have indeed grown more fully valued, but their growth potential should give investors pause before they consider selling.

DuPont has fallen 1.3% even though the company said it would work with energy specialist Navitas Systems to look at various safety solutions to try to prevent fires caused by lithium batteries. Given the number of high-profile incidents involving battery systems catching fire under certain circumstances, coming up with a solution could be a profitable niche for DuPont. Yet the bigger question is whether the venture will produce the high margins DuPont has sought lately, and if it won't, it could easily end up being part of what the chemical company chooses to jettison in its quest to concentrate more on the areas with the biggest profit potential.

UnitedHealth has declined 1.1%, adding to its modest losses from yesterday after the company announced disappointing guidance for next year. Due to Medicare Advantage-related cuts, the health insurer said it expects faster growth in 2015 than in 2014. More information from investor presentations today could help investors decide whether the unfavorable conditions are likely to persist or give way to faster growth in the future, but for now, shareholders are choosing to take profits in what has been a very good year for UnitedHealth.

Fool contributor Dan Caplinger owns shares of Walt Disney. You can follow him on Twitter @DanCaplinger. The Motley Fool recommends UnitedHealth Group and Walt Disney. The Motley Fool owns shares of Walt Disney. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.