i didn't disconnect the terminal when i removed my rad (convenient way to drain!!) but i would disconnect it whenever working on my car, because of 2 factors:
1 you may find a wire somewhere that will come in contact with your hand, or short and fry something, and
2 "friends"/family thinking "ooh, I'm just going to go and start (your name here)'s car!!! (that was inspired by a dream i had about adjusting valves on my mbz, and my brother tried to start it, crank, metallic grinding noises, and pow!! that probably wouldn't really happen, but its a valid point, and makes me wonder if I'm more paranoid about my car than i think!!)

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion