For a belt driven wheel you should use 0 for smoothing (you already have smoothing naturally by the belt). I always used 0 for smoothing for my belt driven Fanatec CSRE wheel. For old gear driven wheels like Logitech G25/27... maybe use smoothing factor 2 or 4 to reduce rattle (and only if rattle is a problem for you, like too much noise racing at night). For DD wheels i don't know but i expect 0 too (i never had a DD wheel, only had old gear driven and belt driven wheels).
You have to know, the more smoothing you use, the more you water rF2's excellent and most detailed FFB in simracing market. So only use smoothing >0 if really needed (like too much rattle, noise ...)
EDIT: wheel I use now: belt driven Fanatec CSRE upgraded to 10Nm motors (using 0 smoothing). wheel I had: gear driven Logitech G25 (used 0 smoothing, sometimes 4 smoothing at night cause of rattle)