Do PCs, like cars, need oil and filter change? Sure, unless you, like many people, replace whole desktops or laptops every two years. But even if you do, you will have measurable benefits by doing regular maintenance.
The efficiency of air cooling systems using fans and heat sinks degrade with time. Most PCs have mechanisms to slow down the processor when over heating occurs. Typically mobile processors in laptops slow down significantly after long use, especially with the fan grills blocked. Slow down support is built into mobile processors. Desktop processors didn't, not until the latest ones. You also need system board level and operating system level support. So, if you keep the fans and heat sinks at top form, your PC runs fast and runs safe.
One more thing you may need to do is to reapply thermal paste between the CPU and the head sink. The chewing gum like paste that comes with new CPU's last for years. But once you disconnect the CPU from the heat sink, the paste will be very bad to reuse. So if you build your own system, you get only one chance if you want a good thermal contact. They are not supposed to be good anyway. Circuit City has a lot of more fluid like thermal compounds on sale, which claim to be better. But I just discovered that they don't last that long. But replying them is trivial.
Other maintenance include cleaning the monitor and keyboard. They are hardly life threatening, unless you have a wife like mine. I have quality screen cleaning fluid and micro fiber cloth for my expensive monitor. Normally it's quite dusty as I believe cleaning cause degradation. I only clean it when I need to calibrate color for example, or to view some very high quality image, or just it's too dirty to look at. But once I left my monitor alone for a while, my wife cleaned it like windows, not the software kind but the glass kind. I was horrified by the marks left on my dear monitor. I could image how she cleaned windows, spray on some Windex, and rub it hard with a towel, OMG. Luckily, after I wipe the screen repeatedly with my cleaning fluid, there didn't seem to be any harm done.
When do you need to do maintenance work? I had no choice because my system board alarm went off due to over heating. Lesson number one, you must turn on the temperature monitor alarms. In the past, I use the system board software utilities when the board is new, to verify that everything is working and no overheating. After that, I tend to ignore those utilities because they are quite primitive software from hardware manufacturers, a pain to use. But I learned to keep them on when the system boots, it will be useful years later.
My CPU reaches 60 deg C whenever the anti virus software is scanning my disks. The CPU usage is only some 50%. So I know there must be something wrong. Lesson number two, on a brand new PC, record the fan speed and temperatures when your PC is doing something heavy continuously, such as anti virus scanning, and playing videos. So when your fan speed or temperatures are significantly higher at the same conditions, you know it's time. Or, you can just wait for the alarms to go off.
I clean the heat sinks on the system board using a can of compressed gas duster for computers, which you can find in hardware stores nowadays. I also use it for all the fans in the power supply and in the case. Otherwise, it's hard to wipe the fans clean. I also use the duster to blow away dust from the system board components, and the keyboard. These are hardly necessary but feel good.
Don't mix thermal paste, old and new. The thermal paste that comes with retail processors are hard to remove completely. But those semi-fluid thermal paste are easy. You just need rubbing alcohol and they disappear completely. Then you reapply a drop or two of paste. These paste are usually contained in a tiny syringe, sufficient to be applied for a 100 years. But you are supposed to store them upright. I did keep the syringe upright after I was done installing my new system.
By the way, hot laptops cause low sperm counts, or something like that.