Hey cjo,
yep, the feed dogs are the teeth and when you do free-motion quilting (or darning) using the FMQ or darning foot, those teeth need to not get in the way. On some machines, they can be dropped below and on some there is a cover plate to simply cover them. Last night at my class, the teacher said that even one of those thick paper inserts from magazines can be used to cover feed dogs. (just tape it to the bed)
Now when you use the walking foot, the dogs need to stay up because you want to transport all the layers at the same time. So for any straight-line or grid quilting, the dogs stay in place. Hope this helps.