+++++++ Always good practise. The alternator is connected to the battery through the main 120 amp fuse. If the battery is not disconnected and you short the alternator + terminal to anything metallic while unbolting the terminal on the alternator, the 120 amp fuse will blow; but, during a dead short the fuse will allow much more than 120 amps to flow before it pops so your wrench and whatever it touches will do a very good imitation of a DC arc welder. It won't be pretty and may initiate a fire.
