I use a digital hygrometer, I have found that the cheapo mechanical hygrometers for under $10 are less than worthless. Mine is made for instruments and includes a tuner, metronome and maybe something else. Cost around $50. Once the RH reaches below 40%, in go the Kyser soundhole humidifiers. I also add a little Herco Guardfather humidifier in the neck compartment, but it probably does not do much. My guitars are kept in the case all the time. less damage potential.
Do tops crack YES, the old Harmony has a severe top crack that resulted from living in MN winters without a humidifier. Back when I bought it people were less aware of the problem (or didn't care) in my era of cheap student guitars. Now that i have moved up a notch in quality, I wish to avoid problems.
Oh yes, use distilled water in your case humidifiers, and let them dry out before refilling (but monitor them!) much less mold potential.
Harmony H-173 bought in 1960 (retired), Alvarez AC60S 2008, Eastman AC320ce 2008 "Hybrid", Guild GAD-JF30E(blonde) 2008, Guild D25M 1974, Martin Grand J35E 2009, Martin D12-20 1970
"I'm glad there are a lot of guitar players pursuing technique as diligently as they possibly can, because it leaves this whole other area open to people like me."
Richard Thompson