Email etiquette: Do I need to put a greeting in every email when going back

[Me]

and forth? I send a email requesting information regarding bussiness matters, the person replies, I have another question, he replies, I need some clarification on his answer, and on and on. The first email I address "Hello Mr. Doe:", the second one I did "Mr. Doe:". What if the correspondence continues with just simple single questions and clarifications, is it necessary to each time start the email "Hello Mr. Doe" or can I just ask my question. What about Signatures, should I always sign my name?

[SamLee]

A signature is your choice, I personally always do one.
However, you do not need to write Mr. Doe, or Dear Mr. Doe every single time

-Sam-

^that's a signature i use on Y!H

[Madworld711]

i used to. but, then I noticed they weren't doing that. so, when i am finishing up my correspondence and it's just simple sentences I do not do that. but, I still put thank you.