The Haynes manual was written around a 2002-2003 vehicle before GM took out the drain plug to save a dime. Besides, if you remove the cover, you have access to the magnet so you can clean that off of magnetic particles (worn off gear surfaces) and make sure it's all tiny particles and nothing larger has broken off.
I agree it's critical to make sure you can remove the fill plug before pulling the cover. Put it back in with some anti-seize to make the next time easier.
Edit: CRITICAL WARNING. The cover bolts were put in at the factory with loctite. Be very careful removing them, and you might need to do the "back out 1/2 turn, put back IN 1/4 turn" trick to get them out safety. The reinstalling torque spec is tiny - 20 ft-pounds. Use the removable loctite to put them back in. The fill plug also needs very little torque - 24 ft-pounds. It's above the fluid level, so there's no critical need for it to be totally seep-free.