Rand doesn't know; even his explanation of why he did this did not include greater knowledge of the mechanism. He just said Graendal was so dangerous that killing innocent souls to get rid of her was worthwhile. He's not the Creator, doesn't understand the higher laws of reality and neither are Nynaeve or Min. Truly, I think Cadsuane hit the nail on the head when she backed Min up - if Rand himself utilized balefire in such a way that reality wa destabilized and the Bore was torn open, it wouldn't matter if the balefired souls would be Reborn in one, two or whatever Turnings of the Wheel RJ specified - the Dark One would have destroyed reality in any case, and it would be Rand's fault.

I mean, technically, carrying your line of argument to it's ultimate conclusion, it wouldn't even matter if you balefired a Forsaken because they would come back, anyway. They'd be removed from the field, but there are quite a lot of twisted entities such as Shaidar Haran or Slayer that technically speaking, are much more difficult for a Third Age channeler to comprehend or deal with.

I basically think, in the context of the overall story, that the WoT world is largely a post-apocalyptic one, and destabilizing the Pattern in any way when it is already frayed to the breaking point is not a good idea.