I do this as well. I use a red permanent sharpie to draw the line on the ferrule.
Use a straight edge to ensure that the line is straight. I actually have a sand paper tip shaper that has both the curved side and a flat side. There is a small groove that runs the length of it. The groove lines up to the right height of the ferrule and as long as you don't wiggle, the line will come out straight.
Once you get used to having it, you probably will decide to keep it.
For me, it offers 3 things.
First, I mark the ferrule along the line of where my shaft appears to be at its strongest point which to some degree does cut down on some deflection but not all.
Secondly, since I can't see exactly see where the actual dime or smaller radius is contacting the CB, it still allows me to fine tune where I'm striking the CB much like a front sight on a rifle.
Thirdly, as Randy G states, if you twist the wrist, you'll notice it immediately since the line will turn to the side.
Even though I use a permanent marker, occasionally part of or almost all of the line will fade. Possibly due to the movement of the ferrule in between the fingers during a PSR or using a closed bridge and sometime over a period of time of putting into my case.
When I see it fading, I just re-draw the line.
Sorry. I'm not an instructor (yet) but thought I'd chime in anyway.