My guess is that more basses are repaired because of "impact damage" than climate. If the wood is aged properly, it should be stable. On the other hand if you force dry it with a central forced air heating system, it is going to crack. I don't do that either. I use a humidifier in the winter. This coming winter however it will be in a new environment with radiant hydronic heating and I will not add supplemental humidity. And for my house AC would be more like $100 / month. Still, $$ has nothing to do with it in my case. Keeping an instrument in a house that gets up to 90 degrees or close is not the same as putting it in direct sun where the heat can build to well over 140 degrees.
Heat is really interesting to study. I re-cambered a bow 2 years ago using only the heat of the sun and a black plastic tube. I built a jig that compressed the bow to the shape I wanted, put a dark room thermometer in the bore of the frog end and stuck the whole assembly into a black vinyl tube and put it up on my roof in mid August. Within about an hour, the thermometer was reading 145 degrees F. I left it there for another hour and the bow was re-cambered. I had recently re-glued the bone on the bow tip also with hide glue and it didn't come off and hasn't yet.
Consider this;- If your bass was imported in a steel shipping container that plausibly sat on the deck of a barge, how hot do you think it got in that container??? What was the humidity at sea level???? If it was imported via air cargo, what was the temp and pressure at 29,000 ft. in an uncontrolled airplane cargo bay???
Mine sat in a warehouse at some Pacific port (humidity????) for two weeks or so while US customs did their drill. The box my bass came in said "Musical Instrument - Handle with care". It didn't say ship only in a refrigerated truck. Obviously, careful "handling" is the larger concern. It came to me on a truck. No special insulation in the truck, no special cushy suspension, just a truck. All the way from California to Atlanta. All the way across Texas (some of you might have visited Houston in the summer?) Arizona, etc. In a couple of days of shipping it crossed every conceivable climate zone except polar and tropical. And it probably was close to polar where it crossed the Sierra Nevadas and close to tropical if it went down I-10 through Texas.
I figure at some point, the wood needs to breath to age normally. I don't want to slow that down. If it cracks, I'll fix it. Climate control is a joke, IMO. You have to take the bass from one place to another and it inevitably is going to get exposed to temp / humidity changes in the process. Common sense would be not to leave it in the direct sun. But perhaps in the building process the ribs could be bent that way. I might try it because it definitely worked with the bow.
