I think using stencils or rulers to create brows are acceptable for beginners, but then quickly become a crutch. Very few faces are symmetrical, and measuring out equal distances from a middle area will most certainly result in brows that do not look quite right for the face. If a therapist new to Microblading needs some confidence in the initial treatments, by all means get it from rulers, But with experience they should be able to draw perfectly balanced, proportioned brows that are right for the individual's face structure... freehand!
I don't think it's a lot to ask. When did you ever see a top makeup artist use a stencil or ruler when drawing brows on their models? It's not one-size-fits-all.
When we train we allow our newbies to draw brows in boxes for a few days, to help with their brow balance and proportion. But when we move to real (3D) faces, we push them to use only their eyes. If they can't create perfect brows each time, it's not up to our EverTrue standard.
-- Ramon Padilla