If you put on special dress for an activity, don't skip the helmet. Same as driving, actually: people who don special driving kit wear a helmet with that, everybody else drives without.

When I spent time in a French hospital after bike helmet use (not involving a car by the way, except for the ambulance that called the helicopter), I was really curious if I would continue that pattern or become of of those "helmet even on civilian clothes rides" people. Was expecting the latter, but nope, would still feel as alien as putting on a helmet to drive.