I've always wondered the same thing, before I use ozone, my mastering program, should I bring the mix up to clipping? I have found that the less I the loudness maximizer in ozone has to work, the better the mix sounds, if I get past one third of the maximum setting it just doesnt sound the same. So I've realized it makes the most sense to get your main mix as close to zero as possible.
Though I would make sure that everything is clearly below zero, dont turn it up just a bit and risk clipping, I found I used to do this.
Eric