With the LFM-1 and LFM-2, you really need to get the sub elevated enough to give the driver and ports room to breath. If you have deep carpet, I've even heard of folks putting a board under the unit to keep the carpet from interfering with the driver (although blocks under each foot would probably also work without being as noticeable).

In general terms (subs other than the Outlaw units), I don't think that Outlaw's ever stated a recommendation - can't think of a case when they would have needed to. If your sub was transmitting excessive vibrations to the floor, spikes might help. Of course, spikes might just channel the vibrations to smaller points on the floor (the energy's got to go somewhere, after all). Personally, I'd go with the manufacturer's provided feet for a sub unless I had a problem.