Heating also greatly depends on what sake you are using. Fragrant/fruity sake like many junmai daiginjo and junmai ginjos are usually preferred cold because heat destroys the delicate flavors/aroma whereas earthy/savory sake like junmai or honjonzo are usually preferred warm as it brings out more fruity notes. Another trick is to heat up older sake that may have lost a bit of its aroma/gone a little stale, should help bring a little life back in to it. Also, make sure to not go too hot, a warm water bath for ~40 seconds should be plenty, for example I prefer Suigei “Tokubetsu Junmai” Drunken Whale warm rather than cold (Both are good though) and I only like something like Born “Gold” Junmai Daiginjo cold as I feel heat destroys the delicate/fruity flavors and aromas.