Robert.....the basic answer of when do you require a humidifier is: (and this is not Kazz being a smartass either)

If you own an acoustic guitar.......

Now the cheap laminates can live without it....but any quality acoustic is going to have at bare minimum a solid top.....even better guitars will have a solid top, back and sides.

I found this Tech Tip on Sweetwater's website via a Google Search: