What's good humidity?

What's a good level of outdoor humidity to not really have to worry about cracking in an all-solid ukulele? Would 60% on average do it?

That'd do it. I wouldn't worry too much unless it get much below 40% RH.

So I don't need a humidifier at all?

Not at 60% you don't. If things drop below 30% - 40% then a humidifier is good. At 60% you wouldn't particularly want the humidity to go much higher anyway.

What instruments like even less than very high or low extremes of temperature and humidity are sudden changes. Any time it seems that things are very unsettled I keep mine in cases to give a bit of a buffer against rapid changes.