Everyone has a different perspective on tea and water temperature. The Taiwanese use boiling water on their Oolongs, the British swear by boiling water on their black teas, and the Japanese use barely warm water to brew their finest Gyokuro green tea. Here are some suggested, generalized tips for knowing when your tea is ready for brewing.
White teas: 165°F (Well before it boils)
Green teas: 170-185°F (Just as steam begins to leave the spout of the teapot)
Oolong teas: 180-205°F (After cooling for a few minutes off a boil)
Black teas: 205°F (After cooling for a minute off a boil)
Pu-erh teas: 212°F (When boiling)
Depending upon the specific tea, the volume of leaf and the length of steeping, you may wish to play around with temperature to understand its effect on the resulting brew.
Why is this important?
Since the goal of conscious brewing is to bring out the best qualities of a given tea, it is often advised to try to stop the brewing just as the tannins develop enough to give the tea a nice finish. If the water is too cool, no tannin will be released, resulting in an incomplete flavor, an empty spot in that tea's particular flavor profile. As water temperature increases, so will the amount of tannin in the brew. That tannin can dominate the flavor and other elements will be missed. In some cases only a taste of bitterness remains.
Why are green teas better with a lower water temperature?
With a less oxidized tea, the lower temperature is more prone to provide a complex and full flavor. Japanese green teas, in particular, are very raw—like fresh garden produce. And, as with produce, if you put boiling water on it you will cook it. The cup will seem like cooked vegetables rather than an elegant sweet, light beverage.
The body, or viscosity, of a green tea results from dissolved particulate matter in the cup (such as miniscule hairs and leaf matter). If the water is too hot more acids will be released, destroying this matter and reducing the body of the tea.
Why are black teas better with a higher water temperature?
The more oxidized a tea is, the more stable. Hotter water is required to bring out the tannins of the tea into the cup. If the water is not hot enough, the brew will be weak and lacking in body.
One good experiment is to take a Japanese or Chinese green tea and try the same amount of leaf and same steeping times, but with different temperatures. Sip the resulting brews side by side and see if the difference is noticeable to you.