I always believe opening the casing (opening the side panel) while it's operating is the best cooling method but this will produce 2 disadvantages:

1. Fan noise generated from your cpu and power supply.
2. Collecting dust.

So to overcome these 2 problem, u can always change the cpu or power supply fan to a after market low rpm fan, and then while the PC is not operating u can temporary close the side panel to avoid dust.