Guitars can suffer warped necks and sharp fret ends due to lack of humidity. In all honestly it depends on the instrument itself. Is it expensive and worth keeping a humidor? If it's an expensive acoustic or even an electric I would suggest it. I have around a $1600 PRS electric and Taylor Acoustic and I do not have them protected as such, and I live in Florida where it can get quite hot. I keep it around 70 - 75 degrees and it does just fine - then again, there is a lot of humidity in the air down here. I have never had issues although I have heard from people traveling to extremes (hot and cold) issues can arise. 

i've come to generally think about this as - if "you" would be happy there, the "instrument" (of any kind) will likely be happy as well. 

So, trunk of car August/February all day - probably not, or on (again car) the back seat in August/February in full sun coming through the window - probably not, or etc. People like (?) 70F ish and 50% relative humidity +- and consistently so. 

One thing I know as a builder of instruments of all kinds for a long time now is - consistent temperature and humidity. If it's constantly shifting in temp and moisture, it will eventually come apart, as with some stringed instruments, cello, violin usually made with organic glues, or even modern electric guitars with modern glues and materials - frets will pop and other. 

A good, inexpensive thing I do, esp with my Cello (any "wood" instrument) - in the Instrument Case, I put a slightly open zip lock bag with a well dampened, but not "dripping wet", not even a "wet" - sponge. They last a long time that way and keep the humidity ("wood swell") fairly consistent. If concerned with any "water" smell issue a (1) drop of bleach or likely wise (careful not to discolor anything) so "ammonia"/some soap - again a drop. I actually just use tap water, plain - but just saying, pending your environment. 

I will be leaving a cheap electric that I bought this year in Arizona down there for the summer. We set the AC to come on at anything over 90F, so we'll see how this particular guitar fares. It's a Squier by Fender 'Paranormal' Telecaster with P-90s...not a bad guitar for the money, but I confess I bought it used off of FB marketplace mostly for the color and the P-90s.

I've been told that in the desert (dry and HOT) that solid wood guitars have issues...best to get the laminate acoustics which have more forgiveness regarding extreme temperatures. Not sure if that's true, but a guy who owns a guitar shop in Scottsdale told me that. 

D'addario makes some handy little humidifier packs for guitars. Check those out. I bought a Taylor 314 CE from Sweetwater last year and they through those in for the shipping. That's where I was introduced to them and have been using them ever since. There are also other choices out there on the market, as well as the old case humidifiers you add water too. Just try searching on Amazon for "guitar humidifiers" and you will get lots of choices. 

It is more about humidity rather than temperature. The best is between 40 and 50 percent (which is influenced by temperature). Anything outside this range can have an effect on the wood. Too wet and the top will warp and buckle which will move the bridge changing intonation and string height. Too dry will cause the wood to contract risking cracks in the top. The neck can also go out but that is less likely as many guitars have a truss rod in the neck.

A check is to lay a straight edge across the top of the guitar a bow up is too humid a bow down is too dry. That said, a warped top does not mean the room humidity is off. There are other reasons for a top to warp. 

Where to store the guitar? Some say in a case with a humidity control device. If it is dessicate, be careful not to get it to dry. Me, I leave my guitars out on stands. There is little risk of accidents in my house, the main reason for keeping the guitar in a case. It is too humid here and my house leaks so yeah, many of my guitars have slightly off tops.