The temperate of the water when you pour it over the leaves really affects the way tea tastes. You'll want to use the best quality water available and always start with fresh water (so please don't reheat previously boiled water as it will taste flat).
Black tea, Herbal or Rooibos – heat water to a full boil (212° or lower depending on your elevation)
Oolong tea – heat water until just off the boil (approximately 195°)
Green or White tea – heat water until "steaming briskly" but not boiling (approximately 175-180°)
Mate – heat water until steaming but not boiling (approximately 150° - 160°)
If the water has already reached a boil but you want it cooler, you can wait a few moments until the temperature comes down a bit and then pour it.