Customer testimonials are important in the fact that the allow you to point to already satisfied customers and to allow perspective clients to see for themselves how good and caring you really are.  But I would try and not to solicit these because they may tend to read contrived and pushed.  Just tell your clients that is a possibility if they don't already know.  The best example I give is that when I am done with a job I give a folder with the invoice and a post card they can fill out plus the possibility to do it on the site.  I would obviously prefer the site but I display all the replies in my office good or bad.  I also leave all the comments up good or bad on the website.