I think it's pretty uncommon with completely dry herbs. If there is any moisture left in the flower petals then I'm sure they could rot. It kind of depends on what you're using and how you're using it I guess. Dried parsley leaves in your soap are unlikely to mold. I've seen some people put entire rosebuds on the top of soap and I would think that unless they're somehow preserved that they would continue to degrade. Soap has a high pH which resists funk in general. There was a thread a while back about a famous soap seller who makes MP and puts all kinds of stuff in them. Cherries I think it was specifically and they did get moldy.
