In reply to bodovix:
My big toenail has come and gone a fair few times over the last 18 months. Each time I've expected it to have a negative effect whilst wearing climbing shoes, but each time I've been pleasantly surprised by how little difference it's actually made. The main thing that made a difference to me is that usually the toenail loss was related to running long distances, which in turn meant that my feet had often swollen up to twice their size, which was a much bigger issue as far as climbing shoes were concerned!!
Hope it all works out for you and that it doesn't have too much of an effect.