At least once a year or more frequently depending on useage. The dollar bill unit should be cleaned and lubricated. Depending on use some manufactures recommened cleaning it everytime you restock the machine, but most people can easily get away with cleaning it every $500 to $1,000 dollars run through.
Any time your vaidator begins having problems accepting dollar bills or to extend the life of the unit it's a good idea to clean them. Just tear open the envelope and stick the presaturated card into the machine. Wait for it to spit the card out and insert it again, several times. Each pass cleans and lubricates the belts, pinch rollers, and optical bill path.