If it's constantly below 20%, you might want to get a humidifier cause it definitely affects a guitar over time.
In winter, the humidity here goes down to about 17% (ie, your lips hurt and your voice gets so scratchy you can't stop coughing.
When I clean the fretboard on my 75 Gibson now, the neck dries out so much it almost turns white.
I've got electric heat so I've got humidifiers going all winter.