Not only every record but everytime it's played. At least in my experience this gives the best long term results. any dirt, even surface dirt, can be ground in by the needle. If the record is cleaned befor play this reduces the possibility of this. To me, it's less of a sonic benefit than a long term, record lasting benefit (as records are expensive and sometimes difficult to replace). Some people use a carbon fiber brush for the "in between" deep cleanings. This dry method gets rid of most of the loose dirt, but I still prefer using the machine to clean each time--I guess it is obsessive compulsive at that point.
Is it necessary to clean every record?
I'm constantly buying used records, some of which are in excellent to mint condition. I put them in an ever growing stack of "to be cleaned" records, some of which are now several years old, because I have a rule not to play records that haven't been through the VPI machine. Is it really necessary to clean the ones that appear as if they've never been played? I listen to a Planar 25, Classe integrated, and Spica TC-60's. I'd appreciate your thoughts.