Soak clams with cornmeal: yes or no?
According to Ask A Chef the cornmeal is not necessary.  It is the salt water that actually causes the clams to purge themselves:


  The thought behind using cornmeal is that it is an irritant that
  causes the clam to "cough" and expel all sand that may be in its
  shell.  There is also a thought that it helps to change the flavor of
  the meat and make them less fishy (I can't say I have noticed a flavor
  difference between a clam purged with cornmeal and one without).
  Whether you use cornmeal or not you will need to use saltwater. 
  Soaking the clams in saltwater for an hour or two will allow them to
  cycle clean saltwater in and out of the shell and purge the sand. 
  Using cornmeal to purge is common and is thought to aid in the
  complete expulsion of the sand, but it really is not the real reason
  the purging will happen.


As far as purging farm raised clams, some (perhaps even most) suppliers purge them for you before sending them to market, so doing it yourself may not be necessary.   However, I could not find an authoritative source that indicates this is always true.