Rituals are always cast with a stress die. No question about it. The rules are clear on that, both in the section on rituals and the section on mastery.
But it also says under mastery that if the magus is relaxed they do not roll botch dice "even in a non-magic aura or when using vis", which back in 2012 suggested to you that if before considering the aura or vis the botch dice are reduced to 0 there are no botch dice, even for ritual spells. (GoF Magical Obj Penetration)
So,as I see it, with mastery you still roll a stress die for rituals in relaxed situations (as the rules require), but "there are no botch dice, even in a non-magic aura or when using vis." (ArM rules, p. 86.)
Specifically, it says:
As I read that, it says a few things:
1.) You add your mastery score to your CT;
2.) You subtract your mastery level from botch dice when you have to roll botch dice (e.g., casting in a stressful situation);
3.) You always cast mastered spells (including mastered rituals) with a stress die;
4.) If you cast in a relaxed situation with a mastered spell, there are no botch dice,
So, basically, if you roll a 0 on the stress die when casting a mastered spell (Formulaic or Ritual), that provokes you to have to roll your pool of botch dice. But as noted by the citation above, your pool of botch dice in this case is zero botch dice, so you don't have to roll any.