While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Apple (NASDAQ:AAPL) slipped 1% this morning after Wells Fargo downgraded the consumer electronics gorilla from outperform to market perform.

So what: Along with the downgrade, analyst Maynard Um maintained his valuation range of $536-$581, bracketing the current stock price of $561. While momentum traders might be attracted to Apple's share-price climb in recent months, Um believes much of the bull case is already baked into the valuation.

Now what: Wells sees the possibility of continued pressure on gross margin, as well as a tighter market opportunity for Apple going forward. "Gross margins have decreased by an average of 225 basis points (bps) in the period following the launch of new form factor iPhones while increasing [nearly] 225bps in the two quarters following an 's' launch," noted Wells. "With the secular story, in our opinion, largely over, we believe the stock may be more susceptible to trend with margins." When you couple those competitive headwinds with the stock's solid rebound since late June, waiting for a wider margin of safety seems prudent.