If you don't want to take your system to pieces then I would recommend using a paint brush to remove the dust and dirt, a vacumm will do fine to suck it up.
NOTE: The paint brush must be made with real bristles, NOT nylon (get one with a wooden handle too if possible). Static and PC's don't go together. Likewise don't touch any part of the PC with the vacumm nozzle. Be carefull of jumpers, make sure they are fully seated. If they come off first you may have to sort through the hoover bag and then you've got to figure out where it goes. Yes I speak from experience.
I have used this method for years and it seems to work fine for me. Stipping down machines is all well and good if there is only one or two, when your cleaning a couple of dozen it becomes a little long winded. Also cans of compressed air cost money, one paint brush should last you years.
As for physically washing a PC yes I've had a friend wash dried on orange juice of a board, dried it under a hand drier and back in and working within a couple of hours on a critially important system. However only ever done as a desperate measure. On a parting note many years ago a slighty intelligence challanged customer after being advised to clean his hard drive (I hasten to add not by myself) did just that. Then couldn't understand why after stipping the drive down, washing the parts and reassembling them it refused to boot up. No urban myth, just born and raised in Deal, Kent, UK. If you know the area you know what the people are like.......
Regards,
Gavin Coles