is this nescessary?

There is nothing like a give and take relationship here. But as a mutual understanding if you keep on reciprocating the favour your friends have done, it will help to earn more replies if not money or star. They would definitely be interested more, to reply to your discussions.

It isn't essential to reply no. As far as I know it doesn't affect your money, but if people give you +ves for your replies then it can affect your rating. I reply sometimes as I want to know more about what the person has said, or just because I need to explain my point better as they didn't understand it.