Let me be more specific:
1. Google clearly states: "It is not only the number of links you have pointing to your site that matters, but also the quality and relevance of those links."
Webmaster Help Center - Link schemes
Are your clients web designers? SEOs? In other words, are their sites relevant to your site's theme? If not you are asking for trouble. You can avoid that trouble if in those links the "nofollow" attribute is implemented.
2. It is maybe not unethical, if you do not require that. But anyway I did that once, but never again. Some years ago, a client screwed their site I made, and they still had my link on it. That sucked! I had to hire an attorney to convince them to get rid of my link. Are you sure that your signature would be worth there? Me not!