Solid body guitars by their very design are resistant to warping. You're really only concerned with hollow body instruments with really thin flexible wood for tops. Guitars are very susceptible to humidity changes, Ukes less so. That's because guitars have more strings, more tension, larger pieces of wood. It's relatively easy to bow up the top of a steel string acoustic guitar. Although the tension is less, classical guitars are susceptible too due to their much lighter bracing and construction. To me, Ukes are more immune to rapid humidity changes than the guitar family simply because they're smaller and with only four strings, have lower stress on the face of the instrument. This may not be true of high end ukes that have been designed very lightly to get the most sound out of it. I'd pretty much say that if your uke is $200 or less, not to worry about stringent humidity control. I'd rather have my Uke by my side than stashed away in a case for fear of it self destructing. One day I might just open the case and find broken, corroded strings from not allowing it to "breathe" naturally.
Some people damage fine instruments by being too obsessive over humidity. Once they learn that it *can* have an effect, they go to extremes with wet sponges or expensive machinery and such and end up making a mess of the wood. One thing worse than dry wood is soggy, moldy wood. The builders typically keep their instruments in the 40 50% humidity range so it's right in the middle of the typical household readings. The wood has been stabilized for long periods of time to acclimate it before cutting. Making drastic changes after they're built just makes things worse. Unless you store your uke in a dry sauna and then take it into a steam room to play it, I really wouldn't worry that much about it.
I'm sure Luthiers will disagree (I'd really be surprised if they didn't) but we're lucky to be playing one of the least costly to own and maintain instruments out there. Don't go to any extremes and you'll be fine.
