Summary:
I build my guitars to withstand a wide range of relative humidity conditions, but you should humidify your guitar if the air is very dry.
Luthiers such as myself are highly tuned into controlling relative humidity. We carefully choose what humidity level to keep our workshops at, based on the range of humidities we expect our guitars to be exposed to, and to some extent, on the woods we use and our personal opinions about the inevitable compromise between expansion under high humidity and contraction under low humidity. This often leads us to set unrealistic expectations for players, who cannot control humidity so easily.
Ideally, a guitar should always stay in an environment within perhaps 10-15% of the humidity at which it was built. However, this is simply impossible for most people. Furthermore, it may not be a good idea to keep your guitar under such controlled conditions if you can’t always keep it under such conditions. Suppose you keep your guitar in a perfectly controlled room at 45% humidity, and you have an outdoor gig in the summer. You take it out, only for an hour, into a hot summer day with 85% humidity. What will happen? It will immediately go drastically out of tune, and it will continue to shift over the course of the hour, so in order to keep it in tune you’ll have to retune constantly. Furthermore, the action will quickly go up, so you’ll find it increasingly difficult to play over the course of the hour, and the intonation on the higher frets will go sharp, even with repeated tuning. To some extent, these issues are things we just have to live with as musicians (if you want, you can get a carbon fiber guitar), but you can make your life a lot easier by loosening up about humidity and keeping the guitar in an environment that gradually changes with the seasons. Of course, most people do so anyway, even if their luthier has told them they shouldn’t.
For me as a luthier, giving this advice comes with a risk, however. If your guitar cracks or buckles, I can’t just blame you. The reality is that wood expands and contracts quite a lot under changing humidity, and the resulting structural stress on an instrument can be quite high. Instruments will sometimes suffer the effects. My old cello has many repaired cracks (I haven’t bothered to count them) and still sounds wonderful.
Generally speaking, low humidity is more of a hazard to a well-built instrument than high humidity. High humidity causes the soundboard and back to flex outward, raising the action and causing distortion around the bridge, headblock, and other fixed components, but the guitar usually returns more or less to its previous shape when the humid conditions pass. Low humidity, on the other hand, causes the wood to contract, and the resulting internal tension will be released by cracking if the conditions are extreme, or if the guitar isn’t carefully built to withstand dry conditions.
Luckily, the amount by which the dimensions of a piece of wood change for a given change in relative humidity is less at low humidity than at high humidity. This means that a guitar that’s built at a fairly low humidity (I set my shop to 35%) is unlikely to crack unless the conditions are quite extreme. In New England and other seasonal temperate regions, this is most likely to happen if the guitar is kept in a room with forced hot air heating or another kind of heating that dries out the air a lot, though it’s possible that air conditioning could also dry out the air enough to be a problem. A small humidifier can solve either problem, and will probably make you more comfortable as a bonus! If you live in a desert region that always has low humidity, I can build you a guitar at a humidity lower than 35%, which would be unlikely ever to crack, but would be more likely to suffer damage if it’s ever brought to a high humidity region.
Likewise, if you live in a tropical or subtropical region, your guitar may never be exposed to low humidity. In this case, it may make sense for me to build your guitar at a higher humidity than usual. Such an instrument would require great care with humidification if it were to be brought into a low humidity region, but will distort less under humid tropical conditions.
Finally, if you travel to places with different climates, or otherwise expose your instruments to extreme conditions, and don’t want to have to worry too much about humidity changes, there are things I can do when building your guitar to minimize the risk. In this case, I would recommend certain woods that move less than others, including Port Orford cedar, Alaskan yellow cedar, mahogany, and/or torrefied woods. I would also recommend a thicker sealing coat of shellac or another film-forming finish on the inside of the guitar, which can slow moisture transfer with the air and buffer against some of the most rapid changes.