Yes, you are correct. That said, the difference between idle and max cpu isn't that great. 50 watts or so isn't negligable. I suppose that most folks might see a rise in their home bill of 25 cents a month or more. If it's that critical to them, it's important they don't add folding.
I once calculated it (for Seti@home) at about $7-$8/month for for 24/7 crunching on three machines versus 24/7 idling. Of course, I drive my boxes pretty hard anyway - I do a lot of video encoding and such - so my machines are hardly idling 24/7 anyway, meaning the difference was pretty negligible in the end. I suspect that spare cycles tend to be scarcer for me than a lot of folks, but YMMV :)