Haha... your website is called 'Cycling without a helmet' but here I would recomend it. Not because of the danger of falling, but because of the low overhead. If you would ride a recumbent that wouldn't be a problem though... except when your getting off the bike. ;-)
Short video here (Vlaanderen Vakantieland page 36, grotbiken);http://www.een.be/mediatheek/tag/17?page=36VlaVak has many bike trips in Belgium, many beer related, too.
Helmets are required when cave biking!!! Head scraping is a certain danger. Other rules included, no smoking or consumption of alcoholic beverages. Heh. The guide actually told us that. Who would drink beer while cave biking anyways? That's insane! Hmmm, maybe in Belgium :)