Actually as for Ghaz's Waagh invul save he may call it at ANY time except the first turn, and lasts for the remainder of that turn and the following one, so unless charged in turn 1 he will be able to call it at will. ( see Ork codex
pg.58 Prophet of the Waaagh! )
I still side with ole Ghaz, if for no other reason than he is the coolest of the super baddies here.