Guitar humidifiers - when are they needed?

[player***]

Thanks to this thread my Martin is still curved whew! I need a new dampit though the Planet Waves thingy seems too little to do a good job of moisturizing.

wish you were looking for one a few months back.sold it to someone for less than I paid for it and it had a built in hygrometer to boot still have a case guitar humidifier,not sure how it works so do not use it.PM me it can be yours if you want it.