The urban legend of placing flowers in warm water is being debunked in the professional flower industry. Flowers like water and warm water breeds bacteria faster than cool water. Think about placing vegetables in the refrigerator…it makes them last longer than putting them on the warm counter top.
Cooler water, specifically ICE water (about 36-40 degrees), is best. You can even add ice to your flower water to cool it down. This prevents bacteria growth, and your flowers will still open fully and actually last longer than when using warm water.