I think it depends on what kind of machine you have. You should be able to find it in your manual if you can drop the feed dogs and how. I have a Bernina and there is a square button you depress at the side of the machine; same side as the on/off button.
I too have read that a card over the feeds dogs works but have never tried it. I think I saw on Alex Anderson's Simple Quilts a lady that said she quilted with the feed dogs up.
Judy