With that little topography change, I would be very skeptical of using WAAS for elevation mapping.  Several years ago I did exactly what you are talking about in that I averaged several passes over a field and used that to create 3D elevation maps.  I think I used MapCalc to do the mapping.  I have fields that have over 50 feet of elevation change in one field.  The WAAS maps gave a fairly accurate view of the general elevation features, but they were not nearly accurate enough to do drainage work.  The slope that you are talking about is well inside the error range of the WAAS signal itself.  Even averaging the maps is not going to give you enough detail in this situation.

I have since started using an RTK signal.  There is all the difference in the world in the elevation detail between WAAS and RTK.  If you have more variance in your elevation, the WAAS maps might give you a decent general elevation model, but with 1' of slope per mile, WAAS will not have enough vertical resolution.