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

What: Shares of Valero Energy Corporation (NYSE:VLO) fell about 2% this morning after Bank of America downgraded the petroleum refiner from buy to neutral.

So what: Along with the downgrade, analyst Doug Leggate reiterated his price target of $63, representing about 12% worth of upside to yesterday's close. So while momentum traders might be attracted to Valero's price strength over the past six months, Leggate's call could reflect a growing sense on Wall Street that its valuation is becoming a bit stretched.

Now what: According to B of A, Valero's long-term risk/reward trade-off is pretty balanced at this point. "From here, we believe upward earnings momentum that has underpinned a threefold jump in sector valuations has stalled," said Leggate. "From here, we recategorize the US refiners as a trading call reprising the traditional characteristics of the sector and drawing a line under the structural rerating associated with a US crude advantage that has largely played out." When you couple Valero's hot stock price with its hefty debt load and volatile business model, it's easy to understand B of A's cautiousness.