Compressed air is fine to use and I can't see how its dangerous at all. The majority of compressed air tins blow out air when you hold them the right way up and liquid CO2 (I think!) when you hold them upside down and spray. Just air is the recommended option although the liquid stuff evaporates so quickly it probably wouldn't do any damage anyway, as long as you obviously ensure that your PC is turned off and unplugged!
General things to watch out for are to hold the fans still when you use the air so that they don't freely rotate and damage the motot. Many people recommend against using vacuum cleaners due to the potential for static electricity. There's not really an awful lot you can do wrong as long as you're sensible and use your common sense Best of luck!