A decision about whether to install a dehumidifier to supplement your air conditioning system depends on your climate zone and how effective your air conditioning system is at limiting relative humidity. I'm in zone 4 (coastal VA.) It gets hot and humid in the summer. My air conditioning system works to maintain relative humidity at or below 50% the vast majority of the time. For our two "shoulder" seasons where neither heating nor cooling is demanded (and thus no potential for the air conditioning system to dehumidify indoor air,) I have a dehumidifier integrated into the air handling system on each level. This is primarily a safeguard against mold/mildew which grows on everything here including slow golfers.

A decision about whether to install a humidifier depends on your climate zone, your comfort, and the construction of your home. Introducing moisture in a poorly insulated, poorly air sealed home in a cold climate zone could result in condensation in stud cavities and your attic and do more harm than good. Proceed with caution.