So here's a case where a stress die from Spell Mastery is clearly not beneficial, if going by a strict reading of the RAW.
If you have Magic Addiction, spell mastery is not desirable. The text for Spell Mastery clearly states that the die used is a stress die, even if the caster is relaxed.
The text for Magic Addiction states: "Whenever you use a stress die in spellcasting, you must, whether or not the spell succeeds, make an Intelligence + Concentration stress roll, against an Ease Factor of half the level of the spell"
So, by RAW, if you have Magic Addiction and have mastered a spell, you have to check for succumbing to addiction. However, I don't believe this is the intent of the authors, I use it here to point out that authors sometimes write something that is clearly against their intent. That it has bearing on Spell Mastery and the use of the stress die just lends a bit more evidence that the term stress die used in relation to the Ritual Magic section is unclear as to the intent of the authors.