Djokovic, we've already seen, definitely has lead at 12.
Berdych, looks like theres lead at 12 in this pic.
Del Po, there's a possibility he has lead at 12, cant say for sure.
For a lot of these pros, 2-3 grams of lead at 12 oclock can make a world of difference to their shots. So it's not unlikey, that many players have done this type of customization.