What I've found is that the slabs tend to build up nutrient levels within, so regular flushing is good, and I use Clearex.
Funny you should mention Clearex, I was
just talking about the stuff a page or so back, if you missed the luvfest.
The bottle says the ingredients are a couple of sugars, so when I saw a Botanicare booth at a show 5 years ago I asked them if it was just sugar water. Of course they said no, that it contained many other ingredients, they mentioned the osmotic pressure, and also that it unlocked any nutrient compounds that are unavailable to the plant. Anyway, I still use the stuff, I figured that the sugars dissolved the salts.
If the stuff has sugars in it, that's plenty of reason, certainly for me, to not put it in a hydro system at all, on the grounds that they're food for pathogens, which I don't want in my system and am trying actively to kill with H2O2. Beyond that, sugars are not solvents for anything, much less for residual nutrient salts. Far as I can tell, the stuff can't do anything that flushing with a weak solution of nutes couldn't, if one were seeking to prevent damage from ionic compunds imbalance across the root barrier.
I think ceestyle has it spot on when he says that the conditions that this osmotic pressure imbalance are supposed to cause, bacterial and fungal infections due to damaged cell walls, don't seem common in week 9 amongst those growers who are flushing for a week on plain water. Given the sugar content of Clearex, it seems likely that it actually could
contribute to the conditions it claims to prevent.
potster, I know what you're saying about nute salts accumulating
on rockwool. It has a strong wicking action which carries more water to the location where it has been voided from, either the rootzone or the top surface of the media. When it does so, it carries along more nute salts. In the rootzone, nutes are picked up by the roots. On the medium's surface, salts are left behind and accumulate when the water evaporates. This effect is highly localised, only to the very top surface layer of the medium. Salts don't generally accumulate deep
in a mass of the medium, rather only on the top surface, where nothing is taking them away.
One way to avoid the crust of nute salts is to never allow the upper surface of the media to be wetted. This may not be practical with RW slabs due to their size and more linearly fibrous nature than RW floc, which doesn't wick as strongly and directly as does slab/cube RW. When I was using pots packed with RW floc only, I packed my pots deeply and watered
only from the bottom so the top surface layer, about the top inch, never was wet. This meant there was no nute solution ever exposed directly to air on the top surface of the medium;
voilà, no more nute salt crust.
However, since switching to Fytocell, which must be saturated fully from top to bottom before use, the nute salt crust is back on my media tops. However, since Fytocell is stark snow white, the salts on the surface are less visible- and don't bother me as much.
The plants sure don't care.
Mind you, I suspect they WOULD care if I top flushed a bunch of accumulated salts from the surface down into the rootzone and didn't manage to leach them fully from the rootzone. It'd be a massive dose of nutes- that very likely would induce a burn. SO, what do I do about it?
NOTHING! 
I just leave it be.
