depends on ur location, i c ur in san diego... so i wouldn't know. but if it gets hot yes. assuming u have a case, a humidifier wouldn't hurt.
i live in vegas, and the air quaility here is so dry. i brought a guitar up from hawaii and moving from a humid to a dry place warped my guitar neck. I would definitely recommend a humidifier. I used to put a damp sponge in a ziploc bag and poke holes in it. and stuck it in the sound holes of each of my acoustic instruments. then i ended up buying one of these for the room.
You can make one - there is a Uke Minutes on it:
http://ukuleleunderground.com/2008/02/11/uke-minutes-4-diy-ukulele-case-humidifier/
Also, you can find them online. I usually like looking on ebay or amazon.
Use weather.com to check the humidity in your area. If it's below 35% you should use a humidifier. If you travel with your uke you should bring one too since you never know what the weather might be like where you are going.
just looked up the humidity in my city... well, let's say I almost need a dehumidifier^^ it's 94% and temperature around 40° (felt like 34°...)
oh, it says kapaa, HI, has 100% humidity, does that mean it's flooded???
Traveling shows with ukes all across USa in varying climates i can say humidifiers are a must in some areas. Now if you have had your ukulele for years and its had no problems then leave it as is because it has acclimated to your areas climate However ukes like 45% to 55% humidity and if they can be kept that way morethe better, i have seen fretboards shrink and bodys crack overnight when a ukulele accllimated to that RH region is taken to say Las vegas where Rh is 10% at times. Better safe than sorry...room humidifers are also great if you have many and want to display them if they are strong enough to keep the room constant.