Having had a battery go BANG, shower me and my other car in acid, while throwing bits of plastic past my head at high speed back in 1988 when I first started welding cars, I always disconnect the battery simply to get it well away from having any chance of a stray spark causing an issue - the battery on that car was disconnected and covered up . . .
Fuel tanks - depends where I'm welding, Close to it, I might, especially with plastic tanks.
Fuel tanks - depends where I'm welding, Close to it, I might, especially with plastic tanks.