That would only be necessary if the filter mount would turn with re-focussing.
The quality of my Nikon 25-105 zoom lens is not at all bad, really. The only thing that bothers me is that it needs readjustment of the polarizer whenever I change the focal length, because the filter mount turns with the zoom - but not with refocussing.
> Do circular polarizers need to be adjusted every time the lense is> re-focused or can they just be screwed on the front of the lense and> left?
No , they need to be rotated each time , as witha linear one , until you can see the darkening effect on the sky , or reflections disappear , or whatever you are trying to achieve with it . Remember , they work best when at rightangles to the sun..... Steve