It may not be hes mandatory pose but its definately his signature pose. Every great Olympian has one. When ever you see a bodybuilder perform that pose you instantly think of Sergio Olivia. Its a pose in which your body looks best, arnold, dorian, haney, coleman etc all have a signature pose that combines their muscularity, symmetry and conditioning.
I think it was the 2005 olympia where Gustavos routine included all the signature poses of all the Olympia title holders, very good routine.