If you put your Macintosh notebook to sleep, everything except the RAM goes to sleep. Newer notebooks (some PowerBook G4 versions, all MacBooks) write the content of the RAM to the harddrive as well, in order to be able to resume even after the battery has completely lost its juice and the RAM has been powered down. So when you close your lid, your notebook might still write something to the harddrive - and I wouldn't throw it in the backpack for, say, another 30 seconds.
But, quite certainly, I never shutdown my MacBook. The sleep function works very well, and it means that I don't have to re-open all my applications and find the documents that I was working on. Lets me keep open my environment, which _greatly_ helps efficiency.