Just heading into autumn here, which is one of the times of the year I "air" the instruments I have that spend most of their time in cases.  I usually do it mostly in spring and autumn, when temperatures and humidity aren't too extreme.

Basically keeping them out of their cases for a couple of weeks.

Years ago, when I was first starting out in music, the owner of a small music shop I often went to told me it is good for instruments.  She had a very small shop and nowhere near enough space to have all the guitars and etc out hanging or in stands all the time.  So some of them spent most of their time in cases in her store-room.  So in spring and fall, she'd have cases propped open all over the place, and would also take the opportunity to rotate her stock.

She claimed that long storage in even perfectly clean dry cases could still harm particularly wood instruments.  Said she'd seen some go brittle and fragile with too much time spent in the cases, so she considered it necessary for them to have some airing out time so they stayed in good condition.

Some years I helped her, in exchange for a couple sets of strings or a discount on some instrument or accessory that caught my eye.  It was a very good time to be a customer there, since you would see things you never even knew she had, and often she was in a mood to offer a particularly good price, if you looked interested in something. 

I don't have near as many instruments as a shop, but still enough that I don't have enough stands and etc to keep them all out all the time.  But when temps and humidity even out a bit in spring and fall, I do usually bring the case instruments out to get a couple weeks of air, and also to clean the cases and etc.

I also clean the amps and other gear, wipe down cables and etc. Basically get everything looking as perfect as it can.

I got to thinking about the wood this year, though.  I'd do the twice a year "spit and polish" anyway.. But I've often felt that some of them do sound better after even a day or so in the open air.  I think that it may give them a better chance to adjust to humidity or something. 

I usually also take cases outdoors for an afternoon in the sun before vacuuming them out.  Mostly I've done that to kill off any mold or mildew spores, before they can cause a problem, but I've heard some people take violin cases out into the sun to discourage the bugs that eat bow hair, so there might be even a bit more reason to do it with violin cases.  Anyway, a little sunshine and fresh air sure doesn't hurt how a case smells. LOL

I've done it for decades now, never had any problems from it, so I guess it at least does no harm.  I've also never gotten any sort of mold or mustiness in my cases, unlike some other musicians I've known over the years.  But I figured I'd mention it and see if anyone else here knew anything about it or has any experiences related.