There are numerous types of Bentonite. Some may perform better when added pre-fermentation, others when added post-fermentation.
My understanding is that when added pre-ferm, less bentonite is required and less stirring is required because the fermentation will cause the bentonite to circulate in the must/wine. Also bentonite assists the yeast to circulate (provides nucleation sites) and thus helps to create a quicker more complete fermentation.
BTW, bentonite attracts proteins thus reducing protein haze.