It probably depends a bit on the printer driver and app you're printing from, but generally yes. I suppose the theory is you might as well render the page as quickly as possible so the printer isn't waiting for the computer (which I've seen, and it's sort of sad).
Keep in mind that if the priority of the printer task isn't very high, it doesn't mean much that it's using 100% of the processor, as the OS will give priority (instantly, without you noticing) to any other application that asks for it.
This is how SETI@Home works, for example--it's always using all available processor, but will immediately give it to whatever app asks for it.