It really depends on the helmet.
I can't fit a goggle strap underneath my Bern Watts with earpads without dying of a headache so I put it over.
I also have a Pro Tec bike helmet that perfectly fits a beanie and the goggle strap underneath. Sits tight, no slipping. I prefer it this way because you can take the helmet off without removing the goggles or beanie. And yes, I think it looks better. But I wouldn't sacrifice performance/safety for the looks.