Varying anchor text in links posted in a forum

Hi. there are a couple of forums that I have always used personally. They cover topics related to my new website and when I post a reply to these topics from time to time, I use my main keyphrase for the anchor text when i put my website link at the bottom.

Could I vary this anchor text?
Would this have any advantages/disadvantages?


Also, does google differenciate between links posted on a forum with the thread topic content that is not related to the subject area of my site in comparison with the opposite?
ie. if i posted a link here on this post as opposed to a link at the bottom of a post in a forum thread that had keywords mentioned in the text - can google see more value in one than the other?