It does have a bad effect if you link exchange with pron sites and what not. So be careful who you link to. And yes, the more related the other's content the more it helps you, but by linking to websites that you've designed it won't have a bad effect.
I've got a PR of 2 and I'm only linking to 35 of my sites that I've designed.
also, make sure that your link text are keywords that you are trying to rank for. making your link matthaff.com will only help you if people are searching for "matt haff" if you make it "Atlanta web design by Minds Eye Cumming Web Design" then that's focusing on keywords "Atlanta, web, design, minds, eye, Cumming"
get the point?