This may simply be that Japanese green teas are brewed at lower temps and it is not so important to maintain the initial temp. On second infusions, these green teas usually benefit by dropping the temp another ten degrees or so, which may fit in with this explanation. On the other hand, with Chinese teas we often raise the temp on later infusions. In addition, there is less clogging with partially full pots if you pour slowly. Avoiding bruising the leaves by slow pouring and by not swirling the leaves in the pot may also help avoid astringency. I am not sure where this recommendation is from, but it is not Japan. It, of course, depends on your pot, you don't want to make a blathering mess, but you should put enough tea that filling the pot to the top will give you a rich and flavorful infusion. As for the temperature, for any tea of reasonable quality one should start with either cold water, for the highest grades, or about 110, for more average teas. Each subsequent infusion should use slightly higher temperature water, in order to bring out all of the flavor of your tea. There should be little to no flavor left in the leaves when you have consumed your fill of tea. In fact, the leaves should have opened up to the point where they are recognizable as such and can be eaten without any bitter flavor. If there is bitter flavor, find a better source for your teas. With quality leaves clogging is also not an issue as the leaves will be large and without dust.
