Would never wear a helmet under any circumstance in the M3W except when forced to by ACU regs for hill climbs etc.
+1 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?