There is a myth about how one should never prepare green tea with boiling water. This myth is false or at the very least lacks nuance.
Water in tea extracts amino acids, which give tea sweetness and umami. As the increase the temperature of the water we extract more and more caffeine and catechins, which are bitter and astringent.
This happens because amino acids are soluble at low temperatures, while caffeine is not.
Thus, teas with high amino acid contents should be prepared at lower temperatures - such as gyokuro, which is prepared lukewarm or cold - while sencha can be prepared with hot water, although boiling water would make it too bitter.
Low grade teas like bancha, genmaicha, and houjicha have light tastes and are low in amino acids, so they can be boiled.
Another consideration: higher water temperatures allow more intense aromas from the leaves, even if it is at the expense of a sweeter taste. Indeed, professional tasters in Japan use boiling water to detect defects in aroma, taste, and colour.