THC allows people to cut corrugated material and have a warped workpiece or even an unlevel cutting bed. But i'm not aware of any similar technology for waterjet machines. So how critical is nozzle distance from workpeice for quality of cut, etc???? I'm sure it varies with material being cut.
If you are looking to keep close tolerances (like +/- 0.010" and tighter) and you don't want rounding on the top edge, proper stand-off distance is important. The closer you can cut to the material the faster you can cut and the straighter your cuts will be because you will loose less of the stream's energy to the atmosphere. Several waterjet manufacturers now offer height control. I work for Bystronic and we have been using height control on our waterjets since we first started building them in 1988.
Usually some form of abrasive nozzle protection is integrated in with the height control to reduce the possibility of nozzle breaks.