Water temperature changes tea's flavor
If you use high temperature, like boiling temperature, it makes tea more bitter/astringency and stronger aroma. On the other hand, if you use low temperature, such as 175F or 140F, it makes tea more sweet/mild but less aroma. That's why the ideal temperature varies depending on tea's characteristics.
For example, Genmaicha has nice nutty aroma but doesn't have much bitter taste, so many tea companies recommend to use boiling water. On the other hand, Sencha, which has a lot of astringency, is recommended to be brewed with lower temperature.
You also need to find your preference. You may like to brew Sencha with boiling water if you like the bitterness of tea. It's OK not to follow the brewing instruction on a package