I haven't bothered because it adds weight
and won't help much. The vast majority of floor damage comes from ABOVE (leaking windows
, roof vents and appliances) rather than from below.
Frederick, OSB is essentially plywood in smaller pieces and the stuf used in most trailers has waterproof glue, like some kinds of plywood but is NOT particle board. All wood rots under the right conditions, even the treated stuf, marine plywood, etc.