As a SEO, when I visit a site that has a link to their SEO company, I will always spend a few minutes to analyze the site architecture, their incoming links, their content and other SEO indicators.
To me, adding a link to your company’s website on your clients’ sites is stupid for many reasons:

• Any curious SEO will be able to spy theses sites and can try to “steal” your techniques if he understands why the sites rank well in the search engines. Also, if you added a link on every website you optimized, you don’t want people to find them all and see what you’ve done. If you use the same link on each website for example “SEO by XYZ company”, it will take only a few seconds to find your clients’ websites;

• If you have tons of clients websites with a link to your website and all their pages, search engines will probably don’t give any value to these links as they don’t look natural;

• If you didn’t ask the permission to put a link to your website, you just don’t deserve it 🙂

Note: I’m not turning paranoiac and saying that you should not link to your clients’s sites either, but I think that it can be a good idea to only present a few of your “best” clients on your website to appeal your visitors, rather than having a full client list so people can have a chance to analyze all your work.