My general rule of thumb:
Anything that needs to look sturdy and/or like hard material (i.e. helmets, rigid armor, etc.): 6mm
Anything that needs to look tough but flexible (i.e. armor padding, separated armor details, etc.): 4mm
Details on the armor (etching, emblems, raised accents, etc.): 1-3mm depending on preference/piece
The thicker range (4-6mm) hold their shaping better and are sturdier. The thinner range (1-3mm) are easier to carve in detail, but don't retain shape as easily. I agree, 6mm is probably what you are looking for for a helmet, but just thought I'd give a "my experience is..." guide. Good luck!