The water is best freshly filtered and should not be re-boiled because this diminishes the oxygen content.
Generally, for good leaf tea the water should be below boiling. This is because the amino acids (which produce the tea's flavour) dissolve at the lower temperatures. Tea made with water at 100°C will be more astringent and less sweet.
NB - don't try this with industrial tea bags. The delicate, subtle flavours of leaf tea are not there - and it will just produce grey water.
Ideally stop the kettle before it reaches the rolling boil - when the small bubbles form along the sides of the kettle. Alternatively the warm cup infusion method (see below) is an excellent way to cool the water pre-infusion .
If you are a real stickler and want to get it exactly right most white teas and green teas are best at 70°C. For black and oolong teas use water around 85°C. For herbal infusions use 100°C water, and 70°C for chamomile.
If you're a convert there are some excellent temperature controlled kettles available out there.
