no good. suck dust quicker than closed casing. insect and creepy crawlies will camp there. Bad airflow. Generally not healthy for your hardware.

if you need to open your side panel for your system to work then it means your system have heat problem. Cause usually, case close will be able to overcome most overclocking. Only if you are doing extreme overclocking that you will need your side panel open to function properly.

I would rather close my panel and reduce the overclocking than leaving the side panel open and causing problems.