One reason I advise people to buy a desktop computer rather than a laptop if they don't need portability is because it is a lot easier to clean the dust out of a desktop than a laptop.
I've had several instances where a PC was spontaneously restarting that was caused by static electricity in a very low humidity environment. Walking across the floor (carpet) without even touching the PC was enough to cause it to happen occasionally. I attached a wire with an alligator clip on one end to the PC case and grounded the other end to the center screw on the outlet cover and the problem was solved.
If static from the user is an issue, please be advised to wear anti-static wrist guards, and/or have a static-discharge touch pad nearby. Either method will reduce the possibility of the static being disharged into to computer.
Couldn't agree more with DrWho's post about keeping a PC elevated above the floor: the foot-shuffling traffic at that level kicks up a lot more dust than you'd realize.
Thought I'd also mention that if you can you should keep the PC away from doors or heavily-trafficked areas where people go back and forth a lot. My wife had endless dust problems with her desktop PC because her work area is near double doors to the porch that lead outside and this area was also where we all went in & out of the kitchen and dining room. Moving to the far side of the room and putting her PC up on her desk helped tremendously. :)
It depends on the environment. Where I keep my desktop, I see no difference in dust accumulation when it's on the floor or up on my desk. If it's on a carpeted floor, there shouldn't be a lot of dust accumulation but every PC needs to be cleaned at least once a year.
This computers and carpet thing came from the old days when someone in an office would walk across the carpet and touch the keyboard and confuse or damage the terminal. Old solutions were to spray the carpet with a de-static product (usually a soap/detergent), carpet with wires woven into the carpet, or carpets woven with conductive organic materials. With foot traffic, the de-static particles would fall away and you would have to spray again. The metallic carpet fibers could break from wear and be sucked into electronic places. I worked on big-iron mainframes that used suspended floors with special carpeting on them.
Another problem came from plastic carpet-protectors used to keep chair wheels away from carpet. These created a lot of static electricity. Then we had special carpet-protectors that had a electrical connection to drain away static electricity. Shuffling your feet or squirming in your seat can also build a static electricity charge.
The problem is not static electricity entering the desktop PC. It is enclosed in a metal box. That is bonded to the power supply, which is bonded to your power system by the ground/earth pin on the power/mains cord. PC crash/damage from static electricity discharge enters your PC via the keyboard, mouse, USB, monitor, etc.. Touch something that is grounded/earthed before you touch your PC peripherals to prevent this.
The manufacturers use metal in the case to help keep inside the radio noise that is generated by the PC. You can hear this noise on a portable AM radio. Try holding it next to the PC.
I have four PCs that are mounted about a yard (meter) above the basement floor. There is no carpeting in my home. We have circulating hot water heat, not convection heating. These PCs run continuously and I still have to periodically take them outside and blow the dust out of them. Don't let compressed air spin the fans when doing this. The DC motors will act as generators when spun and can produce excessive voltage.
As far as taking the covers off of an operating PC, the only exposed voltages are a maximum of 12 volts. Your automobile battery is 13.6 volts. My portable radio uses a rectangular 9 volt battery. 120 (or 250) volts coming into your PC goes first to the enclosed power supply, then has only low voltage outputs. Before touching anything inside the PC, first touch the PC's metal frame to equalize any static electricity differential.
Now, you do NOT want to tilt an operating desktop PC. If you have a hard drive platter spinning at 7200 RPM and you tilt that, the gyroscopic effect is like a sudden, sharp turn on a bicycle. And you have a read/write head that is trying to glide as close as it can to the disc surface without touching it.
Personally, I feel the interval should be substantially less than yearly; more like monthly or no less than every 2 or 3. Wouldn't even go w/out the few minutes for this, more than 6 months @ the most. It's ok if it doesn't seem badly in need when it is regularly done on a shorter than 1 year term. (Regardless of where it 'lives').
As I said, it depends on the environment. Almost all the PCs I've serviced have never been cleaned and most are more than a year old. Most need cleaning but I've seen some that don't.
They may not, actually, need it; I'm just of the mind that they be, @ least, 'checked' sooner, considering it takes so little time or trouble. That's all.
Opening up the case can lead to unnoticed problems. I once dislodged an IDE cable to the point where it was making an intermittent connection. Took me awhile to figure out why it would boot sometimes and other times failed. I prefer to minimize the times I'm inside my PC. Part of the "if it isn't broke, don't fix it" syndrome. I clean my desktop every 6 months or so. I recommend yearly cleanings to my clients since they can do more damage than good unless they pay me to do it.
Also, I dispute the "it takes so little time" quote. Many desktops are a real PITA to open up and many are in locations that place strain on the external cables when you move the desktop to where you can open it up. I worked on some Dell's that you had to place horizontally and push two release buttons while using a third hand to pull one side up.
Laptops can actually be easier to remove dust from.
The way I have my desktop set up on a stand, it's 3-4 inches from the floor.
It accumulates dust on the gills quite noticeably every few months.
Every six months it needs to be opened right up and cleaned out thoroughly.
One might even have to take the fans out and clean them separately.
This thread reminds me of some photographs I saw online several years ago of the inside of a computer used by a tobacco smoker. I couldn't find those photographs again, but here's something similar. This is the inside of a three year old Dell computer used by someone who smokes two packs of cigarettes a day.