Never open my case, except to add memory. Never saw any dirt inside, everything is gleaming clean. I change computers about every four years. The last one I gave to my granddaughter who had it for eighteen months now, the machine is about five and a half years old.
My motto is " If it ain't broke, don't fix it".
How do you know when to clean your computer?. Is it overheating, some malfunction that makes you think dirt is the problem, some time schedule, or is your machine saying "Clean me, clean me", or are you just bored?.