The problem with the light method is any change in orientation between the light, the rod, and your eye moves the reflection on the blank. This means each time you want to refer to this axis you must do three things 1) make sure the blank is returned to the same position under the light (usually not a problem if you're using a wrapper, just make sure the wrapper hasn't moved on your table/bench), 2) the rod must be rotated so the exact same axis is up (this means you need a reference mark, indexing headstock, or other method to verify the axis) and 3) you must move your point of view to the exact same axis and keep it there as you work (this is a skill that has to be learned with a bit of practice).
Does the above work? Yes. Is it easy for a beginner to do? It can be, but for the beginner it will take some practice.
If you take the time to 1) mark the blank with a scribe or ink in the wrap area and 2) place a reference mark just past each side of your wrap area, the marks will be there as you wrap and you won't have to go through the above steps each time you want to check your progress. As you fill in the wrap area and cover the marks you can use a straightedge set to span the reference marks atop the wrap to check the alignment of your thread crosses as you pack your threads. This gives you the freedom to move about as you work without having to constantly confirm the alignment of the light reflection.
Here's an image: [
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Notice how the wrapper's point of view as they work is not in line with the reflection? The marks on the blank give the wrapper the freedom to turn the rod to face them instead of having to move their body to get the reflection to line up with their work.
