From CE 1913: "Of the musical settings, which are very abundant, the appropriate word must be one of caution in view of the direction of Pius X in his Instruction on Sacred Music (22 Nov., 1903, IV): 'In the hymns of the Church the traditional form of the hymn is to be preserved. It is not lawful, therefore, to compose for instance, a Tantum ergo in such wise that the first trophe presents a romanza, a cavatina, an adagio, and the Genitori an allegro.'
Singenberger, in the 'Guide to Catholic Church Music' (St. Francis, Wisconsin, 1905), gives grade, voices, composer, etc., of more than six hundred setting of the "Tantum ergo" [including plainchant melodies, settings by C.P. Bach, A. Bruchner, G. Faure, M. Durufle, F. Schubert...].
Authorities differ as to the time for incensing. Martinucci directs the placing of incense in the thurible before 'Tantum ergo' and the incensing after 'veneremur cernui'. De Carpo suggests both either before 'Tantum ergo' or after 'veneremur cernui', according to the custom of the particular church."