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 Domino's Pizza (DPZ -2.00%) slipped 2% today after Oppenheimer downgraded the pizza delivery company from outperform to perform.

So what: Along with the downgrade, analyst Brian Bittner removed his price target of $64, suggesting that he sees limited upside and possibly even significant downside from these levels. The stock has skyrocketed over the past couple of years on better-than-expected growth, but Bittner believes far too much optimism is baked into the current valuation.

Now what: While Oppenheimer believes Domino's fundamentals will continue to improve, the firm outlined several reasons why its shares may not:

1. Earnings upside opportunity appears constrained (low operating leverage and a lack of financial engineering catalysts);

2. Comps now enter a cycle of tough comparisons and growth will likely decelerate without identifiable grow-over drivers (such as pan pizza last year); and

3. Valuation (25 times P/E, 16 times EV/EBIT) is at an all-time peak (now in line with DNKN) and multiple expansion from here is difficult to justify following the stock's outperformance of the past two years (+192%, vs. S&P +41%).

Given that Domino's certainly looks priced for perfection, Fools would do well to heed Oppenheimer's warning.