I have been leveling and setting up hundreds of guitars over the years and have been thinking about the validity of arguments for the use of the famous neck jig ( dan erlewine) for some years now. I don't use one myself. A friend of mine uses one and once i levelled a fretboard and was not satisfied and let him do it afterwards with no improvement. Of course this does not mean anything but thats my experience with it.
With my engineering background i have issues with the arguments used for the jig.
In short: i dont think it has enough benefits and when i have a problem neck i prefer to work with the strings on so its not a simulation of forces but the forces really at play.
Here are some of my thoughts and i wonder what you guys think, knowing there is quite some users here that really like it. Maybe if your arguments are good i will have to start using one

1) a guitar strung up has some relief most of the time. During the setup in the jig the trussrod is tightened to level the fretboard and forces are intoduced. Here is my first problem, the forces now are not the same as during correct string tension and relief.
2) to simulate the straight fretboard under string tension the neck is manipulated. The vectors intoduced here are different than the vectors resulting from string tension.
3) in the end the correct string tension is introduced during setup (1st time during process) and some relief will likely be introduced by releasing the trussrod. Again new forces are introduced here.
So for me it would seem that the simulation is not very realistic and work is still done in a situation where forces and vectors are incorrect.
Therefore i think that In the rare occasion that the tension is needed during leveling frets is better to use a l/u/t profile with strings on.
Very curious about your opinions and really wonder if there is someone with much experience that really has seen significant improvement in setup results using this or simular jig.
Grz Mario