You bet you do - at least in the colder months! Come October the relative humidity where we live (Eastern Canada) starts to drop and by mid November it can be far less than the 40% required for safe guitar storage. In northern Alberta it plunges into the single digits. That means trouble. Wood moves as it takes on and gives off moisture. That movement is across the grain. But bracing strips inside the guitar running at angles to the wood grain of the body constrain any movement of the top and back.