Using a humidifier for your acoustic guitar is a good idea. They are inexpensive and, when used correctly, they can save your beloved instrument from disaster. Wood reacts to temperature and climate, and as your guitar adjusts with the seasons and weather, you may start to experience fret buzz or other playability issues.
In worst-case situations, your guitar may even crack or warp. Humidity (or lack of) can really affect a guitar, so taking precautions here falls into the category of “better safe than sorry.”
This is why you often see the acoustic guitars housed in a separate room at the guitar store. Controlling the humidity and keeping it around 50% maintains the guitars in good condition, and you should continue that practice once you get your instrument home.
This is even more important when you live in a part of the world where the air is typically dry, or in the winter months when your home’s heating system may dry out the air.
Do you need a guitar humidifier if you live somewhere where it is already warm and humid for most of the year? I think it’s still a good idea. Air conditioning can affect humidity, and unless you are monitoring the climate in your home to make sure humidity is in the proper range you don’t really know.
So, get an inexpensive humidifier and learn to use it correctly, if no other reason than the peace of mind it provides.