This is just not true. Every wing I build has sheeting from LE to spar and shear webs and I adjust incidence this way. You are correct that the type of construction will affect how much twist can be achieved with covering but if it is anything less that a fully sheeted wing you can get great improvement with the technique I described. The OP was discussing a "trainer style" airplane and very few of those have fully sheeted wings so I didn't feel the need to get into that discussion. I have successfully removed over 4 degrees of twist from this style of wing exactly the way I described.

One thing to be aware of, the amount of correction possible with covering is dependent on the type of covering used. MonoKote has the highest tensile strength of any covering out there and is best for this technique but even the softer coverings will give some degree of correction.

Dave