Hell, you are suposed to use a staic discharge braklet when working on the cars ECM to avoid fryin' it with a static spark.
So YES you should allways disconnect the batt. Neg when welding.
As I tell my students...
You can weld on 100 cars and be fine then fry the elec. on 101.
101 WILL be a Benz or similar $$$$$$ car. (murphy don't ya know)
It's unlikely that you will fry the car, but, it is also unlikely that you will win the lottery and people buy tickets every day
But, if it is your car...... Weld away if you want.
Parts stores have a surge protecter that you can buy for about $40 that you can clamp to the batt instead of disconnecting it.
Cory