Upon reading the documentation, I found that Dell recommended against running the server with the cover off. The airflow was so good, that they were afraid things would heat up without the cover on due to dead spots. This logic may go against the experience that some of us have had. Normally, taking the side cover off of your tower case is a good way to open up the system and keep things cool. The only reason that this is true is because many systems that are built by people tend to have terrible airflow.
Bad airflow comes in many forms.
You can have things blocking airflow such as IDE and floppy drive cables. Fix these problems by using garbage bag ties, elastics or plastic clips to hold your cables out of the way. Another cause of bad airflow is a case that's full of holes. The cover off of the case is the extreme example of a hole, and in some cases, where airflow is poor, it will actually help keep the temperature down. Finally, bad airflow can come from a lack of fans, or too many fans operating in a confused configuration.