Wood is a hygroscopic material - it adapts to the humidity of the surrounding atmosphere by losing or regaining water, which makes it either shrink or swell.

Ideally guitars (as other wooden instruments) are built at a relative humidity that matches, or is slightly lower than the yearly local average where the instrument will be played.
In this way the guitar is able to adapt to moderate changes in both directions.

In my workshop I keep a constant relative humidity of ~ 50%.

While very high humidity will not seriously harm an instrument, extreme dryness will.
In Austria during the cold season in heated rooms it often has 30% or less which is much too low.
The wood shrinks so much that the frets stick out on the fingerboards sides, the neck curve might increase which leads to higher string action, the doming of top and back disappeares (actually can invert) and cracks appear in the plates.
If you expose your instrument to such long dry periods over many years irreversible damage will result.

Therefore it is important to care for a proper humidity (> 50%) in the rooms where the guitar is situated most of the time from the first day on.
A quality hygrometer and humidifier are at least as important as a strong case.
If you have no influence on the rooms atmosphere, for instance at university, a case-humidifier is the means of choice.

My instruments bridges have a two-hole-tieblock.
If you do not tie the treble strings the right way they will slip and hit (and damage) the top.
Please have a close look at the drawing below. If, however, you are not sure how to do it right, a knot at the end of the string will work as fine.

To clean your instrument (any finish) it is best to use a damp cloth (with water) and wipe dry afterwards.
Persistent spots often can be removed with spittle.
No alcohol on shellac, no polishes that leave wax or other things on the surface.

For the fingerboard, use ready-made fingerboard oil/cleaner, gasoline or pure alcohol. Be careful with shellac polishes!
Damp pot cleaners (the coarse side!) are also very suitable for this task - they can also be used to clean the frets.

Shellac is a great finish for a guitar, but there are some things you should be aware of:

Compared to other finishes it is very vulnerable, Be careful with fingernails, buttons, zippers, etc.

Shellac is sensitive to heat - a hot car boot in summer is an improper place for an instrument polished that way - the finish would get soft and sticky.

Alcohol dissolves shellac so be careful when cleaning.

Water is not able to dissolve shellac but if it stays too long on the polish it will make it dull.
Sweat: There are some (few) people who will sweat shellac dull in minutes and off the guitar in some weeks, others do not have any problems in years - this is just a matter of luck.
If you want to avoid any trouble put an additional cloth between you and your instrument where you touch it (at the chest and under the arm that rests on the side and perhaps top of the guitar).

It is possible to bring a dull polish back to high luster and to touch up a stronger damaged surface to some degree. You also can build up a complete new finish but it takes time and therefore is not cheap.
It makes sense to be cautious.