Note that when commercial movers say they will put everything in "climate controlled storage" that means that the truck will be unloaded and the stuff will be repacked into a "storage crate" at the storage facility. You have no assurance that the "fragile" stuff that the driver and his crew placed carefully a the top of the load won't end up in the bottom of the crate, or even that it will all get into the crate, since you won't be there to watch.

When you are ready for delivery, the "crate" probably will be unpacked and repacked into a different truck, although sometimes at the destination the crates themselves are loaded.

You will not be present at either of these unpacking/repacking unpacking/repacking operations. The "assurances" that the movers give about how carefully things will be handled are mostly bullshit. The consensus opinion of those who actually do these operations (they're called "lumpers") are that this is where virtually all losses occur. While "dishonesty" is not any more common than in any general group of workers, I've known a couple of "exceptionals" who've had fairly nicely furnished homes from the "losses."

If you read all of the fine print in the shipment contract, you'll find that the carrier is relieved of virtually all obligation for damage or loss, unless you make separate and very specific arrangements for individual items, and if you ship a guitar and receive a ukelele they may not (voluntarily) recognize that as a loss.

(At our last commercial move, we received a carpet (9x10) and an end table that weren't ours, and two of the four chairs in a set were a different color than the ones we watched being packed. We still find "missing items" that are on our personal inventories, are known to have been packed but weren't separately itemized on the packing lists, but can't be found - 8 years later, such as specific individual LPs and VHS tapes, etc.)

If at all possible, unless your extra guitars are of "extremely ordinary value" I would recommend shipping something else and arranging to take them with you. An alternative might be to have someone store them at origin (perhaps a friendly music store?¹) and ship them UPS or FedEx (insured appropriately) after you're prepared to take delivery at your destination. Another choice might be to see if UPS or FedEx or another "fast freight" outfit will ship and hold for pickup. (It's much easier to insure for reasonable value on an individual item shipment than for a bulk household move, although of course the bulk rate is cheaper by the pound.)

¹ Even with a good friend, you should have the agreement to store and ship in writing with copies to yourself and the person(s) helping you. You should also make sure they're packaged for shipment before you leave them (unless the agent is better prepared to do it than you are, agrees to packaging, and it's included in your written agreement).

Most of the stuff that goes in commercial household shipments gets there okay. Nearly every shipment has some loss. It's still a judgement call whether a particular item is something you're willing to take a moderate risk with, or whether you want to "do it special."

I'd personally worry more about the "handling" than about the heat for something like a guitar.

John