Canned air is expensive in some places (here in dominican republic) and will not take a way the kind of stuff that gets sticked to the blades. As acidosmosis said, hold fan blades with the fingers and then use a small paintbrush to remuve the crap from the blades, both sides preferably. One the dirty stuff is all over the place, use the canned air to blow it all off.
Dust is dangerous. And it's ugly to see when you have a window. And it's ugly to touch when change components. And can get inside the pci/agp/dimm/cpu sockets creating connection problems between the pins.
Also, if you have led fans, if you don't clean them they will "bright" less.