Once had a situation where a head director made the decision to march the school R13s in pouring weather at a BOA contest. Three of those clarinets had major cracks in them by the end of that night even with drying. The worse part was that the assistant packed some much cheaper clarinets to be prepared for the situation. The head still wanted the nice clarinets on the field. I think it depends on the equipment you're using and the amount of rain as well as the atmospheric pressure. Also, many times parents will fix things for us that we have no idea they did. I'd say that if your students can make great sounds in the brass and the show still works without woodwinds, don't march woodwinds in the rain please.