If you place a link on clients sites your site will become heavy with "footer links" which will probably cause the links value to diminish. It's expected that your site will obtain a lot of different types of links (comments, forums, index, in-content, directory etc) so when your site has mostly one type it doesn't look natural.
Can't hurt, you may see a small boost in rankings, but probably can't help all that much either. I'd do it to see if I get additional direct business from the links but I'd stop doing it if that doesn't happen. A natural link graph is simply the best kind though it takes time.