Making a helmet crest

[Decebalus]

I know the currently accepted theory for helmet crests is the centurion was the only one who would wear a crest in battle.

When should a legionary wear his helmet crest if not in battle?
Why should a centurion wear a crest different fom the the "ordinary" form, when the legionaries wear no crests at all?
In my opinion the crests were made to be worn in battle, confirmed by art and written sources (the incident in Caesar's Gallic war where the Legionaries had no time to put on their insignia), like in many other helmeted Ancient armies.