There are many general rules that people created over time. Tea ceremony has very precise and strict rules to prepare tea both in matcha (powder green tea) tea ceremony and sencha (leaf style green tea) tea ceremony.  These complex array of rules are often intimidating to beginners and casual tea drinkers.  I didn't like the rules either, especially they do not explain why things were the way they were, but if you understand the reason behind it, it is not very mysterious any more. 

We will look at sencha here.

 Sencha is a leaf style Japanese green tea, grown in the sun and steam processed, rolled and dried.  This process is adapted for preparing at cooler water temperature than black tea and oolong.  Since sencha expresses so much astringency when prepared at boiling water temperature, generally water is cooled down to 70 C - 80 C.  This temperature range hinders astringency and bitterness to dominate the taste of sencha, and instead allows sweetness and richness to be more dominant.  (See graph below)

 The graph shows"how water temperature affect extraction of each chemical components". The graph on left is prepared at higher temperature than the right.  Caffeine, amino acids and tannins all extract quickly in a first few minutes compared to the lower temperature steeping.

Flavors 

Generally speaking, caffeine expresses bitterness, amino acids sweetness & richness, and tannins astringency.  High amino acids concentration relative to caffeine and tannins make the tea taste sweet and rich in balanced flavor.  Tea with high caffeine and tannins such as the left graph tends to taste very bitter, even when using the same tea!

Experiment with our tea - Kagoshima First Flush

 Kagoshima First Flush is a fukamushi (deep steam) sencha.  Fukamushi processing enhances sweetness and bright green color, yet the basic preparation idea is the same. 

Our perceptual taste testing shows that cooler temperarture (70C 45sec steep) has more substantial sweetness and richness due to lower concentration of tannins and caffeine present in liquor.