Room temperature water is best because your plants are in that environment already. Changing their environment suddenly can lead to shock. If you were using soil as a medium then that would depend on the container size, plants needs ( strains categorized as indica, sativa, or hybrid indica/sativa ), lighting source, air quality, and temperature. Since I use CO2 enrichment in both vegetative and flowering stages, I find myself watering my plants in vegetative state every 2 days and then feeding them every two days. In the flowering state they require every other day watering and every other day feeding.