i'm one of those people who ALWAYS reccomends using a humidifier with any acoustic instrument. hell, any wooden instrument. it's just playing it safe, and for a way way cheap price.
honestly, it depends on where you are in california (humidity varies - its VERY DRY where i live in central/northern california), and on the wood, etc.
there are lots of factors.
lots and lots and lots.
okay, not that many, but more than one.
regardless, i still would always reccomend using one. just to be safe.