In case I send John Smith an email and he sends me a response, should I include “Hi John,” when I am replying to his email?
I always do. This is a matter of personal preference. Some people view e-mails as electronic letters, whereas others view them as text messages. I belong to the former group, so I always include a greeting like “Hi John,” or “Hello John,” or even “Dear John,” if it’s a formal e-mail. I also always include something like “Best regards,” or “Sincerely,” at the end of the message before signing my name. I’m slightly turned off by e-mails that start without a greeting. Others don’t care one way or the other.
A lot of business e-mails these days will just begin with the name of the person being addressed; e.g., “John:”. I personally find this a bit brusque, but it’s widespread.
Bottom line: whether you include a greeting is a matter of personal preference, and it’s perfectly acceptable to use the same greeting in subsequent e-mails.