I would ask you the following questions:
- Are all the client sites on the same server ?
- Do you have a clients page where you link back to each of your clients ?
- How does your link profile look ? Do you have lots of other links ?
- How many client sites are we talking ? 10/20/50/500/1000 ? And then how would you compare them to the total links you have ?
If your link profile is mostly natural, I wouldn't worry about it that much. But going forward you could nofollow them. Also, what's the anchor text in your links ? Your Company Name or Website Designed ? Do more of Brand Name, combined with Anchor Text. So all sorts of variations. Do not make them consistent.
I hope that helps.