Concerning ukulele's that are solely acquired for collecting, would you loosen the strings? I like playing concert and tenor size, but collect Soprano's. I'm not sure how I should leave them.
But if you insist on not doing so, make sure that they are well-humidified, strings loose, and not exposed to bright light, whether artificial or sunlight. Check your frets and finish at least once every 3-4 months for signs of too little or too much humidity.
Sharp fret ends from fretboard shinkage, they hang over the edge. Ripples in the top or back wood. I don't know about the finish.
Best bet is to keep them in hard cases with a humid pak or a humidifier AND a hygrometer so you know the humidity is within a few percent of your target amount. Some of my ukes do not do well at 50% humidity. I find 45% is about right for all but four of my ukes. I am slowly weaning them down to 45%.