If used with notching, people can gain a bit of an advantage in BnG (and other schemes) by placing markings on/under their monitors, I even did it for a little while myself, and a number of people seem to currently do this. It's especially helpful for 4 second full power shots, petrol, etc, so you can know exactly where they'll land.
This subject is iffy, but I think we shouldn't allow any kind of visual aids for the game, no matter how small or inconspicuous they might be. It's just one of those things that would look very amateur in a live setting, and to me, playing as if someone else is watching is the standard we should adhere to.
Lack of enforceablility doesn't matter much to me; unlike notching, this is at least something we could enforce in a live setting.