From a practical standpoint one should mention it because one is having a hard time discerning mortal and venial sin and the best person to help with that is one’s confessor. And if one is also suffering from scruples then the confessor needs to be able to help with that. And because honestly no one should feel comfortable telling someone else something is NOT a mortal sin. That power lies with the Church in the Sacrament of Confession.
The safest answer all the way around is
“mention it next time you are at confession” Because we should not say “nope not a mortal sin for you” or “you need help with your scruples”
That is not our place and honestly I don’t think people (especially on this site) understand the seriousness of telling others when NOT to go to confession. I don’t want blood on my and my children s hands.