I am also very wary of every little thing in FF games since we all know how big of an impact some inane thing can cause. Though, as far as the diary, it doesn’t matter. You can read it, then simply lie to Pilka, and she’ll still give you the better armor.
Just make sure you DON’T answer that you read it. 😉
It’s all in the sidequests section of the guide under "Pilka’s Request"…or something like that, lol.
Though, I completed this part before I realized it was in the sidequests section, and now I wish I had read that diary, lol.
(Well, whenever that is, given how gosh-darn big this game is.)