commenting has an etiquette that should be followed. if you are leaving valid and informative information and you have a link that is relevant, its okay.
leaving a link in your signature is okay if the owner of the site or blog says its okay. go with the idea that if they dont request you dont, go ahead and leave the signature link.
"popping in" to say great article....here is my link and nothing else- is not okay.
moderating comments is a great idea and i wouldnt run any of my blogs without moderation. not only can you delete the spam that you receive you can also make certain no comments are offensive or abusive.
i believe that comments are important for your community to interact with your blog or hub and it does allow you to improve your content.
follow your blog etiquette and your instincts. if you are not sure about it, dont do it.