Due to the seasonal drop in humidity I inevitably see more instruments coming in for cracks in the top and string buzz problems. Cracks caused by dry air are almost always along the grain lines in the area between the bridge and the butt end of the instrument. The soft wood between the darker grain lines shrinks due to the lack of moisture and separates from the denser and stiffer winter growth. In most cases these cracks can be cleanly repaired with little evidence of the repair, if the instrument is brought to a repairman as soon as it becomes apparent that there is a crack.
Most repairmen charge by the inch for crack repair, so the shorter the crack the cheaper the repair.
Additionally dirt and oil that works its way into a crack over time makes it more difficult for the repairman to hide the repair. Many people who have forced air heating have a humidifier built into their system. For the rest of us there are a couple of solutions. Ideally an instrument should be in its case when it is not in use. This gives it a degree of protection from the room environment.