A question on how to steep green tea?

Hi, I just got a tea catalog and it says green tea should be made with slightly cooler water between 170-190 degrees.....I let the tea kettle start whistling then wait ten seconds then steep my green tea, wondering how much longer I should wait for this temperature....also does it affect it that much? I would hate to think that all the green tea I've had did nothing for my health.....thank you......