Obviously acoustic and classical guitars don't have external pickups that need to be setup, but are there any other adjustments that need to be done on a regular basis? There doesn't seem to be a bolt at the head of the guitar to adjust the neck like there is in my electric, and such. I can't imagine that one should just do nothing and expect the neck to not warp and the guitar to play well forever.
Good question. A nylon strung instrument exerts a lot less tension through the strings than a steel strung instrument, and as a result a well made guitar doesn't need to have the extra reinforcement in the neck of a truss rod (the job of which is to counteract the forces trying to bow the neck toward the fretboard). Having said that, though, if your steel strung instrument needs regular adjustment to the truss rod I'd take that as a warning sign that something is not right (poor quality instrument, exposure to rapid changes in temperature or humidity, etc.) I have a dozen or so guitars that hardly ever need any adjustment, unless I change string gauge.
In short, there are very few reasons you'd need to make adjustments - the art and science of constructing well made instruments has had hundreds of years of development, and most instruments you can buy these days will give many years of service with little or no need to tweak.
Many acoustic guitars have a truss rod that is accessed via the inside of the body of the guitar. There are some guitars that do not have them, and there are also some acoustic guitars that have them accessible via the headstock.
Sometimes the action is so bad that sufficient adjustments cannot be made via the truss rod alone.
If the adjustments aren't too severe, shaving down the saddle can lower the action (if it is too high), and adding shims under the saddle can bring it up (if it is too low).
If adjusting the saddle is not enough, you have to do what is called a "neck reset". This involves having a repair person remove the neck of the guitar and reshape the joint where it connects with the body.
Yes, all guitars need to setup or adjusted over their lifetime. Check the following and have your guitar checked out by someone who knows what they are doing if you are not comfortable:
- Intonation - tune up your guitar; preferably with a decent electronic tuner. Fret each string at the 12th fret (the notes at the 12th fret should be the same tuning).
- Buzz at any of the frets
- Action (string height and playability)
- Quality of frets and fret wear
Proper adjustments over time will never hurt an instrument, and may indeed make them play better and last longer. Each instrument needs to be set up from time to time, and someone who really knows what their doing and can "read" what the wood does or may do over a course of a year will provide a great instrument no matter what original condition or price was paid. A good guitar setup is best viewed as an investment in its fulfilling life.
Often guitarists will perform these setups themselves since they know what tension or size of string they use, how they play, where they play, and what kind of buzz they can get away with (or lack there-of). Major repairs that require special tools are often left for the luthiers and guitar techs.
There is a system in place to keep the guitar's physics from counteracting in a negative way. This is usually, and most importantly the straightness of the neck and fingerboard and its relation to the truss rod, which as you said, has an adjustment at the headstock, or adjustment at the end of the neck near or at the neck heel.
However, with an occasional setup, and keeping the guitar comfortable (meaning, not leaving it anywhere you wouldn't want to be left) in a proper case and humidified environment it will tend to stay exactly in its happy zone for nearly the entirety of its life. It is very much up to the owner just how far it gets out of it's happy place or not though, unfortunately.
Speaking of proper care:
I'm currently working on readjusting and resetting necks in two cheap Epiphones that were haphazardly left unattended and out of their case. The necks or fingerboards snapped apart and the action of the strings - and therefore the delicate balance with the truss rod - had drifted to an uncomfortable zone. And just one unfortunate event caused these guitars to significantly play worse (and therefore "harder") from where they were at before, off the assembly line. With time, these guitars may be back to similar original condition and playable for someone who cares enough for them.