The edgefinder is a bit off a clumsy tool and it takes some work to get teh best out of it.
One thing you need to do is make sure you are using the right width to get teh cleanest line. The second is to feather the results ( effectively blur teh edges of the cutout so its not as rigid.) You can then clean up the edges using teh transparency brush as an eraser.
To be honest the best choice is teh freehand tol. It seem to get much better results.
Take a look at this tutorial here on selection tools in DIP.
http://scrapbook-bytes.com/cgi-scrip...icle_159.shtml
HTH