I have kept corys on nice rounded gravel and they had no health issues, however after having them on sand I would really recommend it for them. The sand allows them to filter and sift the sand through their gills, encouraging their most natural of behaviours.
Some cory species maybe more susceptible to infections if kept on gravel, when my tank had gravel my false julii never did any well, while the peppered corys powered along. Now on sand my false julii are almost as big as my largest male peppered corys, and have never shown the tendancy to weaken and die off like they did on gravel substrate.