Ed, I'd do it at least once or twice a year. If you start to see some dust buildup in the cracks on your front panel, then it's time open her up. The air is being sucked in from the front of the computer. This is where it builds up first. Too much dust can overheat the processor and your computer will frequently crash.

It's fairly simple, take off the cover and get a can or two of compressed air. Don't forget to turn the computer off first!!!  Take all the dust out especially on and around the heat sink on top of the processor. Make sure all vents are clear of obstructions. 

I don't see the need to take it in. You can do it yourself Ed.