It is not necessary to pre-shrink batting; it depends on the end look you want.
It is never necessary to pre-shrink polyester batting because polyester doesn't shrink.
I prefer using cotton batting. My favorite batting for machine quilting is Mountain Mist Blue Ribbon 100% cotton batting. Trying to preshrink this particular batting would be disastrous because it is not needlepunched, does not have any scrim to hold it together, and also does not have any kind of bonding agent applied to the surface to hold it together. Merely soaking this batting in water would result in disintegrating wads of cotton.
Someone who wants to preshrink cotton batting (typically because they want a very flat look to the quilt) would need to use a cotton batting that is bonded, needlepunched, and/or needlepunched through scrim. Even with these battings, you never want to agitate. The usual method is to soak in the washing machine, never allowing agitation (turn off the machine). You can hand agitate by pushing down on the batting with your hands, if necessary. When batting is saturated, advance to "spin" cycle to get rid of excess water. Dry in dryer. Water temperature isn't that important; most of the shrinkage occurs in the dryer.