I agree totally with deigja.

I do not call just anyone claiming to be a dominant Sir. In my opinion the honorific is something I use to those who I know and who I hold in high esteem and respect as a dominant. In practice, this mean that online there are few I call Sir. Obviously my own Dom does get called Sir and he is the only one I submit to.

I have to say, if any man who talks to me insists on my calling him Sir or Master when I don't know them at all, it rings alarm bells in my head.

I think as long as you are polite, there is no specific rule around this, particularly in the online community.