I am going to set links from guestbooks to my site. I want to put about 10-15 links from guestbooks with PR6 or higher.

As I understood it can be useless.
But I want to know can this cause damage to my site?

I personally wouldn't want to risk it...

HL

It can't damage your rankings. If it could, a good way to ruin a competitor's site would be to post hundreds of links from guestbooks.

however, it's probably a waste of your time.

Signing up at forums/sites for the sole purpose of getting a link from a guest book borders on spam IMO.

With that said though, a link from a guest book is just like a link from anywhere else - it "counts".

Most forum owners have started preventing the SE bots from indexing their member lists (in that case the links won't count), or don't show members on the guest list until they hit a certain post count within the forum itself.