I've never actually thought about putting a limit on how far across I will go, but I think Chuck's estimate is a good one.
If you ever watch a game on ESPN Classic from back when they used two officials -- notice how far out on the court those guys work. Watch as Earl Strom comes comes upcourt almost in the middle. I don't try to emulate those guys, but I keep them in mind so I remember that there is no reason I have to be pinned near the sidelines.
I do know that when I go across I am looking to get an angle and give up distance by heading deep, sometimes into the backcourt. Bad things can happen if you are in close, wander across the floor, and a skip pass is picked off six inches in front of you. Let's just say that I learned that first hand about 12 years ago -- my partner and I argued all the way home whether I took that charge (but the coach was not amused)
I won't go all the way to the sidelines. I don't think I need to be that close. Crossing over sides? Not this century for me, unless I'm covering for my partner who gets caught.
Crossing over would bring back memories, though. Remember when we used to have to cross over the court during a live ball to get into the "cadillac" position?
Rich