I know in the old days, leaving your sigs on forums and posting links on Blogs was a useful way of getting links to your site. Is this is a good way or not a good idea any more and even possible penalty by Google for spamming?
As far as the category is relevant, then it would not be considered spamming as it would be helpful. But posts in irrelavant category could affect SERPS i believe.
It shouldn't adversely affect ranking - but rather get completely ignored so having no affect whatsoever. If there is any benefit, then it is very marginal at best.