It is better to let the tank become full or nearly so before dumping. This provides a good flow to flush the tank contents. If possible (i.e. when dumping with a full hookup) I hold the valve open on the toilet for a couple of minutes to flush clean water through the tank before closing the dump valve. I make it a point to do a fresh water flush through the tank at the end of every trip. Regardless, "matter" does accumulate over time and will become odoriferous. Use of tank cleaning products periodically is a good idea. Usually they recommend you fill the tank, add the solution and drive to slosh the contents, then dump.