My "studio" such as it is, is in my basement in one of the spare bedrooms. The basement is finished, but much colder than the rest of the house. I'm not sure how much cooler but probably 2 to 3 degrees. My question is will a cool room temperature be harmful to my guitars? The humidity is fine, it's the temp I'm concerned about.
Depends on the actual temperature. 2-3 degrees shouldn't matter, but be aware that cold temperatures cause wood to contract. This may result in the fretboard shrinking and letting go of the frets, or finish cracks, or just general misbehavior.
But it's gotta be really cold for that - like low 60s/high 50s.
However, moving the guitar between two temperature zones (upstairs for a few minutes and then back down a few minutes later, etc) is not good. Wherever you put it, leave it for at least a couple of hours to let it acclimate, then check the neck.
Cold temperatures, even freezing, usually will not harm a guitar. The wood will not typically contract enough to noticably affect anything.
Newc's point about quickly changing temperatures is the most important issue here. This can cause instant and permanent damage to necks and especially finishes. Most finish checking on anything but true vintage instruments is caused by rapid temperature change. Neck issues are caused when two pieces of wood with different expansion properties (ie, maple neck w/ rosewood board) expand or contract at a different rate...you can see the issue here.
Whenever moving a guitar from one temperature extreme to another, always place it in its case prior to the move. Then let the case sit for roughly 2 hours before opening. This should be enough time for the guitar to slowly acclimate to the new temperature.
I watched an employee quickly open a guitar that came off of a sub-freezing UPS truck. The finish immediately cracked and spiderwebbed all over the guitar as soon as it hit the warmth of the store.
So my guitars sit in my room which is about 62-64 degrees. Does that have an adverse effect. Is that too cold to store guitars and what is ideal? They all (electric &acoustic) are in cases except for one thats kept in a gig bag.
To add; If the guitars are stored at 62-64 and I take the guitar outside to my car (outside is typically colder), then get to my gig site, which is about 70. Would that make a difference, cause I don't have the several hours there to acclimate to their room temp?
I think the guys answered your question, your storage temp won't harm them.
The key is drastic temp change. The fact you have most in cases is best, as the case adds an insolating barrier to temp changes.
I would suggest putting the guitars in your vehicle just before you leave home (and have the vehicle heater going to warm up the insides), then get them out first into the gig site (that makes for the least cold exposure time). Leave the cases closed for a while! It probably takes you at least 1/2 hour to 1 hour to move in and set up amps and everything else, and that gives the case a chance to warm up and the the guitar warmer too. Then open cases and tune up. You'll probably note that they need tuning again in 15 minutes or so, so if you have the time before you play, try to let the guitars out of their cases 15 minutes to 1/2 hour before you play.
In reviewing the above, to be honest, if your vehicle was warmed up to 60-70 and you loaded the guitars quickly into it and out of it, the amount of cold getting to the guitar in a case will be pretty minimal. I don't think you have much to worry about. Its more of the drastic thing of having the guitar in a cold vehicle overnight, then taking it into warm room and popping open the case that's the problem.
In addition to what's been said, remember that 60 degrees is not a temperature extreme. Guitars are plenty happy there. In fact, I keep my guitars in the basement in winter BECAUSE it's cooler. That 10 degree difference also means the relative humidity is higher. The same air at 60 degrees and 40%RH (safe) at 70 degrees is only 28%RH - too dry, especially for acoustics.