Elevating subwoofer about two feet off the floor

Any omnidirectional, long wavelength, (deep bass) is going to bounce around the room creating complex patterns of interference. As you move the source of that wave (your speaker, or in this case, subwoofer), away from a nearby, reflective surface (the floor) you change the frequency and amplitude (severity) of the quarter wave interference.  So, I guess the answer depends on the relationship between your sub and the floor and other walls in your room.  As a practical matter, it's hard to believe that this makes sense unless you have multiple subs and are trying to randomize the sub to wall/ceiling/floor distances.  In that case moving one sub off the floor could make sense.