There are many people asking this question over the net and I've answered this question a few times to other web designers in my local area.
Avoid Site Wides and Non-Relevant Links
Typically any form of back linking that has nothing to do with your niche is less than desirable in this day and age thanks to Penguin and Panda, Website designers should seek relevant links and should move away from having site wide links such as 'Designed by Web Design Company' because thanks to other industry's site wides, and even just one link from an non-relevant page can go against your company.
Strengthening your Keywords that Matter
If you want to strengthen your rankings then if you want to rank better for Web Design topics then you want links coming from actual other web design company's, blogs etc -
There are lot's of web design show case sites and many communitys that hold far more weight than most if not all of your clients will provide, such as:
- UI Parade
Believe in Branding and Mouth to Mouth PR
Personally I believe in branding, having just a designed by blah in the footer without a link will generate some leads if the site is busy - it doesn't need to be a link. Also if you have a good brand and do a superb job then word mouth to mouth will generate more leads than that little backlink at the bottom of the page.
Also if you want relevant links you can publish some of your thinnest work or provide UI Goodies, jQuery Scripts etc and your find that people in the same field will link to your side if your Goodies are good enough....
Oh and remember Google isn't the only way to generate leads :)