Here's a quote from a maintenance guide on storing guitars.
" The easiest way to take care of your guitar is to store it properly. The more expensive the guitar, the better your storage should be. It is generally accepted that the air humidity should be neither too high nor too low, thus somewhere in the 45-55% range, and the temperature of the area should be about 65-75 °F. These two factors are the biggest threat to an instrument, because changes in moisture and temperature can cause permanent warping of the neck and other critical parts. For guitars made out of solid wood, it is advised to use a humidifier to prevent cracks and damage from weather change.
On the other hand, guitars made out of multi-layered(plywood) wood, typically in budget guitars, can withstand relatively more humidity and temperature changes. Keeping the guitar in a case away from direct sunlight can help with increasing the life of the guitar.
The surest way to keep your guitar in good shape is to remember this simple rule: Do not expose the guitar to any climate conditions that you would not want to be exposed to. If you always keep this in mind, then your instrument will likely last years and years.
You should avoid large changes in humidity. Like your body, the guitar gets used to the climate it is in, and suddenly changing it causes stress. Humidity is the most dangerous thing that attacks an instrument, because when wood gets wet, the cell walls become softer and it is more easily bent. Often, the strings themselves are enough to bend the neck. Also, if the humidity stays way too low, then the wood will crack and the structure will weaken.
Temperature on its own is less damaging to the guitar. Wood is generally tolerant to changes in temperature, and for the most part it expands and contracts together. Extreme temperatures, however, can cause serious damage, especially when combined with extreme humidity. Changes in temperature also affects the strings, especially nylon strings, and going into a different environment will almost always automatically detune the strings. Other areas to watch for temperature related damage are any glued joins, like where the neck meets the body, or the fretboard is attached to the neck.
Never expose your instrument to extreme temperatures for a long time. For instance, leaving your guitar in a car in the summer all day, or leaving it outside for the whole night are sure ways to completely destroy your instrument. Also keep the guitar out of direct sunlight as much as possible, because it makes the wood more brittle and it can destroy the color of the instrument.
Keeping your instrument in tune is another good way to make sure that you don't harm your instrument. Strings put certain amounts of tension on the neck which can cause it to bow if the guitar isn't kept in tune. It is also a good idea to make sure that you have a full set of strings on the guitar. "
Now, where's all of the above is very true, I find that the more you play your guitars the better they get, and a little bit of weathering only helps the character and tone. Especially acoustic guitars. This is obviously just my opinion and what I prefer for tones out of an acoustic guitar. If you keep them in a case or out of it I find doesn't really matter. IF the environment is somewhat regular.
One thing to consider if you hang your guitar on a wall is to not hang it on an "outside" wall. They change more in temperature etc.
Hope this provides somewhat of an answer to your question.
Regards,
Henrik
