It's not just OK, it's recommended. Canned air is simply too expensive for anything but spot cleaning. I've been using this 2-stage air compressor
(no longer in production, but originally made by DeVilbiss
for Sears) for 20 years now. I also use (and highly recommend) the VACULA BlackStar blowgun
(with an optional rubber tip), it offers very precise pressure control. In my experience nothing does a better job for cleaning PCs than a good air compressor and blowgun combo.
Note that because of the amount of dust liberated I only do this type of cleaning outside, and depending on the wind conditions (or lack thereof) sometimes I wear a dust mask. When cleaning desktop systems I simply remove the covers and thoroughly blow everything out, taking caution so as not to overspeed any cooling fans. In some cases I remove the front bezel/fascia for more thorough cleaning in that area. I also blow out the power supply from the outside in, and vice versa. Finally, I hold the keyboard upside down and throroughly blow up into and around all of the keys.
For laptops no disassembly is required. The cooling ducts in a laptop are generally sealed such that the CPU fan both draws air in as well as exhausts air out through one or more interconnected ports. Thus if you blow in one port, the air (and dust) will come out through the others. I also turn the system upside down and blow out the keyboard as well.
The only caution when using compressed air (besides following standard ESD prevention practices) is to be careful not to overspeed any cooling fans. On desktop systems I will hold the fan blades stationary using my fingers or a small screwdriver when blowing directly on them. With laptops you can't generally access the fan to hold it stationary so smaller bursts and less pressure should be used to prevent overspeeding. Scott.