Just realized my paper grain mistake after a year on the shelf.
I bound a journal last year and noticed it started to curl badly. Turns out I ignored the grain direction when folding the signatures. The paper expanded unevenly with humidity, warping the whole book. Now I always check grain before I start any project.
For the cover grain, yeah it's super important too. I learned the hard way when a cover I made buckled after a rainy week (the grain was going the wrong way). Now I always match the grain on the cover board to run head to tail, same as the pages. That keeps everything stable when the humidity changes.
My first three books all warped like a bad banana, Margaret. I was so focused on the text block I forgot the cover boards completely, so @wesley966 is right to ask. Now I check grain direction on everything, even the darn endpapers.