We normally recommend grinding carbide back clearance at 12-15 degrees. More than that won't allow enough backing material to provide adequate knife support. Both WKW's and Leitz's systems have pros and cons. The thicker carbide seems to reduce breakage compared to other carbide knife systems. We have customers successfully using both systems.
Carbide is tricky enough to work with.
Adding MDF into the mix only complicates matters more. When using carbide on MDF, the back clearance angle is important. While many people recommend a back angle of 12-15 degrees, I have found that in the field, the use of 18 degrees allows for a better jointed knife and thus longer production runs. If you use wider wheels, the heat build up normally damages the carbide. When jointing any type of knife, including carbide, the correct stone is necessary.