It's time to clear up a common misconception: just because your bottle of sake is served hot doesn't necessarily mean that it's cheap shill. At least, it's not that simple. Until fairly recently, most sake in the US was served hot because of its questionable quality, and this tactic is still employed by many restaurants, as heat masks bitterness and enhances sweetness. Nowadays, higher-quality sakes are far more accessible stateside, but that doesn't mean that the practice of enjoying a glass of warm, cozy sake is obsolete. Keep these guidelines in mind instead.
Full-bodied, less aromatic sakes, like some bottles labeled junmai, are actually best enjoyed warm (around body temperature), as heat brings out their bold flavor and mutes any bitterness that may be present in a robust brew. In contrast, lighter, more delicate sakes (look for the terms gingo, junmai-ginjo, daiginjo, or junmai-daiginjo on the bottle's label) and unpasteurized sake (as indicated by the word nama) are best served at a similar temperature to white wine — slightly warmer than refrigerator temp — as this allows their crisp acidity and aroma to be emphasized. Ultimately, though, it all comes down to one thing: personal preference.