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 The Boeing Company (NYSE:BA) slipped about 1% in premarket trading Friday after Goldman Sachs downgraded the aerospace and defense giant from buy to neutral.

So what: Along with the downgrade, analyst Noah Poponak lowered his price target to $130 (from $150), representing about 5% worth of upside to yesterday's close. So while contrarians might attracted to the stock's weakness in recent months, Poponak's call suggests that much of Boeing's growth prospects are still baked well into the valuation. 

Now what: According to Goldman, Boeing's risk to reward trade-off is pretty balanced at this point. "We believe investors in Aerospace should seek to obtain less exposure to Original Equipment and more exposure to aftermarket," Poponak said. "[T]he current cycle has now driven Boeing to produce 2X the amount of aircraft in did in 2008, making incremental growth in the medium-term difficult." When you couple those growth headwinds with Boeing's not-so-cheapish P/E of 20, it's tough to disagree with Goldman's cautious stance.