To clean, or not to clean?

That is the question. (My posthumous apologies to William Shakespeare)

I've seen PCs that were so filled with dust, animal hair, grease, and loose screws that I was surprised they were still running. There are pro's and con's to do this cleaning, and I've certainly made my mistakes in cleaning computers, mostly due to carelessness and not inspecting the components before diving in. So, here are a few of the things I do in the cleaning process.

1. Check the system event logs, to see if there are any entries in the logs that indicate a component failure. If there is, I point that out and make sure you're aware of the condition, and what the remediation is for that problem. (If the system is still under warranty, either I open a case with the manufacturer, or my customer does that, depending on my service relationship with that customer)
2. Check the BIOS and revew the system environment readings. If the CPU temperature looks a little high, or the system fan RPMs look a little low, the system probably needs cleaning. Heavy dust will put a drag on the fan motors, causing them to spin significanly slower than when they're clean.
3. After determining that the system may need cleaning, I open the case, and visually inspect it for cabling that may be frayed or compromised. If any of the cables look like they were nicked by the CPU fan or some other moving part, I will recommend replacing the cable or the power supply, depending on where the bad cable is.
4. I also check the exhaust fan to see if it is blowing in the right direction (it should be blowing outwards, to draw hot air out of the case - I've seen a few "custom built" PCs with exhaust fans blowing air inward). If that fan needs to be reversed, I make that correction with the 4 mounting screws for the fan. I also check to see if the fan is plugged into the correct place on the motherboard (as opposed to being connected to a power jumper from the power supply)
5. There are air intake slots usually at the base of the front of the PC, under the bezel. I've seen those intakes completely filled with hair and dust. That reduces the intake airflow, and makes it increasingly difficult to have cooling air travel through the case.
6. Carefully observe the level of dust, dirt and grime on the surfaces of the fan blades, heat sinks, hard drive cases, and on the bottom of the case.
7. I have a vacuum cleaner that I use to pull the dust as I blow it out with a duster. Sometimes, it's important for me to vacuum the air intakes on the machine itself, or extract the hair and dust clogs before doing the dusting. I use Q-tips to brush dist off the fan blades when the dust looks really solid, and wooden chopsticks to pull out those clogs, and then I lodge those chopsticks and Q-tips into the fans to immobilize the fans while I'm blowing dust out the case.
8. On a service call, with customers who still have rollerball mice, I open the ring, clean the ball by brushing it off the best I can, and scrape out all the gunk from the wheels and rollers. For the optical mice, I scrape gunk off the pads on the base of the mouse. I also turn the keyboard upside down and shake it out (it's amazing how many staples fall out of those keyboards).

If I'm replacing a component (RAM, HDD, CPU, CPU fan, optical drive), I will generally do this level of cleaning as a courtesy to the customer, and explain why I did it. I don't usually open cases just to clean them, under the "leave well enough alone" rule.

I did a quick search for the different techniques of cleaning computers, and why it should be done, and maybe why it should not be done. Here are some of the different views on the subject:

Cleaning the Interior of your PC
Clean up your grungy PC
In my search for "don't clean the inside of your PC", I did not get a single hit on why it should not be done.