A dry bag can keep it dry after it's already dry....put away dry into a dry bag and it's safe from rain, but put it into the dry bag when wet and you're asking for mildew. The key is to dry the tent out completely whenever you have the chance. You're OK to pack the wet tent in the AM if you have no other choice but if you break camp in the rain then you need to pull the tent out whenever you have the chance and let it air out and dry out. Putting something wet into a dry bag is asking for trouble; it's the downside to dry bag panniers. They can be very useful but need to be used correctly. Rope and a tarp over your tent can go a very long way to keeping it dry. In rain forest areas a lightweight tarp to go over the tent is key.