Not that I don't agree with Herb but....
Temperature plays a very big role. Equipment like a constant temperature.
The wood instruments like @50% humidity.
So if you can control both the better it is for the woods.
So what if don't control these?
The temperature changes would probably not be fast enough to cause immediate
damage such as cracking the finish on a guitar but enough to cause tuning hassles.
The humidity issues will effect certain instruments more than others- such as an acoustic
guitar and depending on the amounts (low or high humidity) and the time of exposure will determine the results.
The results being the wood swelling or drying up...both are not good effects.
Location of instruments also comes into play. A good example is having an acoustic guitar
hanging on an outside wall...the back maybe against a colder temp. than the face.
Easy to imagine it....a cold backside and a warm fireplace front-side...not a good thing.
So personally I'd first have to decide if I was after the heat or the heat and being able to
visually enjoy the seeing the fire.
If it was for the heat I'd probably go with a free standing stove. It would be easier to control the heat and the humidity with it I believe. Just keep it stoked and a kettle of water
on it.
A fire extinguisher, fire/carbon monoxide alarm, and meters/temp. and humidity.....and with friends like mine maybe a few warning signs and possibly some caution tape or railing lol.