Should I clean iMac fan ducts? Should I try to clean the fan ducts at the top and bottom of my five year old iMac? I have seen people say vacuuming or blowing air may mess up the innards.
I replaced some memory in my friend's iMac of 3 year old recently and there was loads of dust on the modules. I blew the dust off the modules but didn't go in to major cleaning as it wasn't my machine. However with my own machine I would be reluctant blowing the dust off the inside of the iMac, but putting a vacuum cleaner at a very low speed shouldn't be harming much (provided the iMac is fully disconnected from the power socket.

Hope this helps.  All the best!