An old Pentium III an extended family member had burned itself out of disrepair. The computer was used for about a decade before being forgotten, but plugged in for some other 5 years. Then they had the bright idea of using the old computer without cleaning it. It all went well for a while it boot up and otherwise was it old win7 self. Except, a couple of days later they let it on overnight. The house owner was awaken alarmed by the fumes of burning plastic and a faint trail of smoke coming from the case.

The over a decade built up of dust had been trapping heat so bad that some old shoddy HDD power cable melted partially exposing the conducting wires. Then it short circuited and arced through the dust setting some lint on fire.

The flame was minuscule and it was controlled fast. But I would say that, it is indeed wise to clean your computers, at least once in a while.