The DV format compresses the video image, and this compression introduces color artifacts which may ruin the color in that carefully lit green screen or around the actor's edges. So, if possible, it's best to avoid DV cameras (or any other equipment that adds compression to image data) when shooting for chroma keying.
If you must use DV, you may be able to eliminate some of the artifacts by blurring the image slightly before keying. The Keylight plug-in has a Screen pre-blur parameter that you could use.
Or try this:
Add an adjustment layer above your blue/greenscreen footage.
Add a slight Gaussian blur to the adjustment layer (two to four pixels).
Set the adjustment layer's blend mode to Color.
Then precompose the blue/green screen layer and the adjustment layer.
Proceed with your key.