Would never wear a helmet under any circumstance in the M3W except when forced to by ACU regs for hill climbs etc.
Sheepskin flying helmet in the winter, bare head with Factor 50 in the summer when warm (surprisingly often) or leather flying helmet when a bit cooler. I wear glasses so goggles tend to go on once speed gets above about 30 or 40mph.
Would consider wearing a helmet where required to by law!
Which raises a thought; could we do with a list (sticky post?) of which countries require a helmet by law in the three wheeler and actually might enforce it for a visitor or indeed a resident?