The drag factor on any erg remains the same at the same damper setting (assuming that air conditions remain the same and the cleanliness of the erg remains the same).
The DF is a function of the resistance that the flywheel is experiencing as it slows down. You can make it vary a little bit with exaggerated pulls but I have always assumed that that is variance in measurement rather than true variation.
Cleaning does have a significant effect on the DF.
You should always check the DF before you row. Many people make the mistake of rowing by damper setting. The DF for any given damper setting will vary across machines and within the same machine over time.
When I first started erging at the gym I was one of those damper 10 guys. It was not until I started reading this board that I got a clue as to how to erg.