hey, my cousin leaves his side off for better ventilation, my research has shown conflicting opinions. some say its good but some also say it disrupts air flow. i have tried without the side on for today and everything seems cool.
5 years ago or so I had a computer that I always left the sides off. Wasn't necessarily because I thought it was better ventilation but just because the side wouldn't go back on very well lol.
That aside though I had that computer for 3+ years or so and never had a problem with it.
I would unplug it about once every 2 months or so and go blow it out and anytime I upgraded it sure was easy to work on lol. Yes it's ok to call me lazy at that time.
goko and instanity are right though. Leave the sides on. They designed them like that for a reason.