The question arrives with every first sake order, and both answers are correct serves. Temperature in sake is a choice you match to the bottle and the meal, the way you would decide between red and white, and the thing worth learning is which way suits what is in front of you.
What sake is, first
Sake is brewed from rice the way beer is brewed from grain, with one twist that makes it strange: the starch in rice has to convert to sugar at the same time the sugar ferments to alcohol, a parallel process steered by koji, the mould that also stands behind miso and soy sauce. The rice is polished before brewing, and how much of the grain is milled away sets the main grades: junmai is pure rice with no added alcohol, honjozo carries a small measured addition that lightens it, and ginjo means highly polished rice fermented cold, which is where the fruity aromas live. Most sake lands around fifteen to sixteen percent alcohol, closer to wine than to spirits.
Why warm
Warming sake, to around body temperature or a little above, rounds it out: the sweetness steps forward, the edges soften, and the smell of alcohol backs off. Fuller, earthier styles take heat best, which is why the classic wintertime order is a tokkuri of hot sake shared around a hot pot, and why most of our sake gets drunk here over nabe and the richer ramen, with a cold night outside and one pot simmering in the middle of the table. It is also the friendliest introduction to sake for anyone who has only met it cold and sharp. The warming is done in a water bath, and the bottle tells you when to stop, heat past warm into hot and the alcohol starts to dominate the room.
Why cold
Chilling does the opposite: it tightens the body and preserves the delicate aromas that heat would cook off. Aromatic, fruity styles, ginjo and anything labeled nama, unpasteurized, want the fridge, since warming a nama wastes what makes it special. On our list the cold side includes nama sake and Ozeki Hana-Awa-Ka, a lightly sparkling sake that behaves more like a celebration drink than a dinner pour. If your bottle has a fragrance worth smelling before the first sip, drink it cold.
What is on our list
Hot sake comes by the bottle, cold nama sake and the sparkling Hana-Awa-Ka by the bottle and the pour, and Kiku Masamune Taku for the serious tables, the full list with prices is on the menu page. Sapporo and Asahi cover the beer drinkers, and free-flow Japanese green tea covers whoever is driving.
How it arrives at the table
Warm sake comes in a tokkuri, the small flask, with ochoko cups beside it, and the pouring has a custom of its own: you pour for the people with you and let them pour for you, a bottle left in your own hand is a bottle waiting for company. Cold sake arrives in the bottle it came in, chilled, sometimes in a wine glass for the aromatic styles, since the fragrance is half of what you paid for. A bottle shares two to four cups over a meal depending on the pours, the 780ml hot bottle covers a table of four with a round each, and a second bottle ordered at the right moment is the oldest signal in dining that the evening is going well.
Ordering without studying
Order by the meal. Hot with sukiyaki, the seafood hot pot and the richer ramen, cold with sashimi, the snack plates and the omakase courses. When in doubt, ask the staff, they pour both all night and can steer you, and the honest rule of the whole subject is that the right temperature is the one that makes you order the second bottle.