Trevor Gore wrote:sides are a lot narrower than backs, so the distance moved is much smaller.
This, I think, is the main reason sides usually don't crack from dehydration, even if the back does.
I sometimes use thicker side reinforcement braces to stiffen squirrely sides, but I make sure they terminate below the top surface of the side, the idea being that even if the sides shrink excessively, the side braces won't start pushing against the top and back. At least I like to extend them further up into the linings, to avoid a weak point between the two, which can lead to a crack.
I remember getting an old basket case guitar to fix up once, it had all sorts of cracks and problems all over. I do remember the side braces and end blocks, with the grain oriented opposite of the sides, were at least a couple of mm longer than the side's width, causing the plates to separate from the side. The glue had also partially slipped from the braces and blocks. I'm sure extreme dehydration and poor material / design choice were the main causes, but it was also a reminder avoid, if possible, structures that resist the wood's movement. That's why I sometimes skip side reinforcements, if the wood seems sturdy, the sides are narrow, and the instrument's outline is nice and curvy (no 'dreadnought style' flat areas).