The on deck batter remaining in the circle until called

[Porch Dog]

From the looks I get from players I believe I'm the only ump in my area that requires the on deck batter to remain in the on-deck circle until the pitcher completes his warm-ups and is called to the plate by me. I've required this since the incident (Christianson?) where a college player was blinded in one eye while supposedly timing pitches while the opposing pitcher was warming up. I used to coach my lead-off batters to do this but I've since ceased to do so. Am I "booger pickin"? Is this a good practice? How is this handled under the various rules? And finally does the age of the players make any difference? Thanks! Porch Dog PS... I do primarily FED ball.