In a traditional 12-note tone row, it seems to me that the first six notes will always either be in the same set class as the last six notes, or will be z-related, but I'm having trouble confirming this. For example:

Schoenberg's Fourth String Quartet:
2 1 9 T 5 3 | 4 0 8 7 6 E

Both discrete hexachords are (014568)

Berg's Violin Concerto:
7 T 2 6 9 0 | 4 8 E 1 3 5

The first hexachord is (013468) and the last is (012469), both have interval vector <233331> and are thus z-related.

Are there exceptions to this? Alternatively, is there a proof of this fact without having to try all 12!=479,001,600 possibilities?