I know that Evaluation and Seal ratings are two completely different things. I took every artifact I could, and didn't come close to my evaluation falling to zero though. It's just nice to have them for when you need to transmute something.

Also, your Seal rating is at the bottom right hand corner of Lenneth's status menu.

As for advice to get the A ending:

During Chapter 4, try to meet Brahms and not fight him, complete the Tower of Lezard Valeth without leaving, recruting Lucian and Mystina, sending Lucian up to Valhalla by chapter six, and not having the Nibelungen Ring equipped at the end of every chapter. Still send at least one person up during each chapter though.