As in the case of notation for the Wagner Tuba, there appears to be no absolute rule as to the position that Wagner Tubas should take on a score sheet. Composers who have written for the work have all had their own ideas including Wagner, Bruckner and Strauss.
Wagner placed the Wagner Tubas alongside the biggest tuba, below the trumpets and trombones, the logic being that he scored for an ensemble of 5 tubas.
Bruckner adopted the same idea as Wagner in his 7th Symphony but then changed his mind in his 8th and 9th Symphonies placing them above the trumpets, as in these works he deemed them to be part of the horn group.
Strauss followed Bruckner’s latter example placing them above the trumpets in his score of the Alpensinfonie and Die Frau ohne Schatten but conversely one finds him placing them below the trombones in his “Elektra” score.