So if for example you are providing someone with some valuable information via an email, they are not likely to consider themselves the recipient of an advert. This second point leads us to the importance of remembering that the reaction to your sig. The beauty of a sig. This is not the case however when it comes to, the people browsing the newsgroup you posted in, and the recipient in the second example above. So now is a good time to look more closely at the content of sig.
Some people like to just have one general signature, which they always use.
Other people have different signatures for different situations. For example, some places actually request signatures to be under a certain length. So if you are regularly posting in such a place, it would be a good idea to have a signature ready that meets their length requirements. I hear you ask, in a tone rather reminiscent of Chandler from 'Friends'. For the sad fact is that you do not have to look very far to find sig.