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 semiconductor company Micron Technology (NASDAQ:MU) closed down 8.6% on Friday after posting disappointing results and receiving a downgrade from Wells Fargo.

So what: Wells Fargo actually raised its valuation range on Micron to $14-$17 (from $11-$14), but with the stock sitting at $18-ish, analyst David Wong felt compelled to downgrade it to underperform. While Wall Street is generally very bullish on Micron's growth prospects going forward, a few firms like Wells have some valuation concerns after the stock's big run in 2013.

Now what: Wells Fargo now sees Micron earning $1.24 per share in 2014, versus its prior view of $1.00, and expects $1.30 per share in 2015. "With Micron's first report following the Elpida acquisition, we now have a fair amount of information with which to model the effects of the acquisition," noted Wong. "However, even our new, higher valuation range is below Micron's current stock price and so we are downgrading the stock to Underperform from a prior Market Perform on valuation." With Micron shares still up about 225% from its 52-week lows and trading at industry-matching price multiples, it's tough to argue with the underperform call. 

Fool contributor Brian Pacampara has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. 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.