Actually no....
It is not done the "traditional" way. Usually, the standby would go on first if the principle was out. If the standby couldn't go on, the understudy would (assuming it wasnt a 'put on' situation.) On the tour for "Wicked," however, it is different.
According to Maria Eberline, her and Victoria are alternated. She said she was concerned when they hired a standby that she would hardly ever get to go on anymore, but the powers that be assured her she and Victoria would alternate evenly in regards to going on if Julia was out (Victoria was put on first preference recently to let her get a feel for the role. The one time Maria went on recently, in Cincinnati, Victoria wasnt rehearsed enough to go on.)
The reason they hired a standby was because JLG leaving created a gap in the track for an Elphaba understudy, so they had to just place in a standby in order to not throw off the ensemble track.
Tonya Pinkins: Then we had a "Lot's Wife" last June that was my personal favorite. I'm still trying to get them to let me sing it at some performance where we get to sing an excerpt that's gone.
Tony Kushner: You can sing it at my funeral.