Well to answer the question directly, it depends lol .. lets phrase this right.. you should not distract the system when its burning, this could cause errors or issues on the target disc. The reason is because the burner flow is constant, usually from a buffer, the burner is not designed to stop for a second, do something else, then pick up where it left off, however, there is a margin here, as long as the actual burning process is not inturpted then you shoudnt run into problems. The buffer keeps information ready for the burner ahead of time.
There is no problem with using resources while encoding. Keep in mind that its probably not a good idea to hog them though , browsing, downloading, chatting on msn would not really effect that with the exception it might take a few seconds longer in the end to complete the actual encoding. If you want to know how much resources are being taken up goto your rightclick the taskbar and goto "task manager" here you can see how much resources a given application is using at a given time. Most apps are resource friendly and use the most power when launching, compiling, converting ect.. some resource hogs are "expecially webcams", some media players, scanning software, and java applets such as yahoo games that have a constant streaming connection.
As a rule I just let the system be when its burning, when its reading I browse, and chat, and listen to music. This way I dont make a coaster, I just restart if I-tunes freezes or something goes wrong.
Hope that helps