Scrum emphasises ease of collaboration over more constrained behaviors that limit co-operation. Formality in clothing or modes of address are unhelpful if they reinforce inappropriate power relationships. That isn't always the case though; if everyone uses equivalent formalities the field may be leveled. Or to put it another way: si tout le monde est "vous", pas de probleme.
In Scrum terms, I reckon it is better to focus on good etiquette than formality. Debrette's says that "good etiquette" is behavior that puts people at their ease...and much depends on context and audience. That's as true in agile ways of working as anywhere else.