My sister's having the same argument with her other half at the moment.
All I know is that I was born two days after my next door neighbour who was my best friend throughout school. We were exposed to the same environments at the same time. My family has a history of picking up bugs easily so was always getting stomach bugs etc up until I was 5 or so. He was picking up the same bugs so his parents went the whole Dettol everything approach whereas I was allowed to get dirty, eat bugs etc. He is always off work with the current bug that's being passed around, whereas I'm only ever off work if I've managed to fall of the bike!!
If I ever have kids they're getting dirty and building up immunities. It's what the body is designed to do.