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 Time Warner Inc (NYSE:TWX.DL) slipped about 1% in premarket trading Wednesday after Cowen downgraded the media giant from outperform to market perform.

So what: Along with the downgrade, analyst Doug Creutz reiterated his price target of $77, representing about 11% worth of downside to yesterday's close. So, while momentum traders might be attracted to Time Warner's buyout speculation-fueled surge of late, Creutz's call could reflect a sense on Wall Street that its valuation is becoming a bit stretched.

Now what: According to Cowen, Time Warner's near-term risk/reward trade-off is rather unattractive at this point. "With shares having run up over 20% on news of Fox's bid, short-term share performance will likely be dictated by how [Twenty-First Century Fox] proceeds," said Creutz. "We still like Time Warner's fundamental position, but see potential upside from any higher bid by Fox as offset by the possibility that Fox will not succeed/and or decide to walk away." With Time Warner shares up more than 25% during the past month alone, and trading at a P/E of 20, it's tough to disagree with Cowen's cautious stance.