FranticFish - 11:49 am on Sep 23, 2010 (gmt 0)
I've always asked for a discreet link on client sites - either from the home page or the contact page (not site-wide). These vary according to the service provided and I also vary the text in them.
If the client has no objection, why should you not get credit for your work? I fail to see why these could hurt rankings in any way - if added naturally, as and when new sites come online.
Not only that, these links get clicked on and have generated new business.
I link to all of my clients too, except where prevented by NDA.