I feel like once the game is finished go for it, otherwise youll be spending more time on these endings than the story itself, plus if you get ideas for endings while working on the story you can just add a note for later and you might be able to expand upon it further and make it better than if you first acted upon it.
Please also make sure theyre very few and far between, it can be a pain and make things less impactful if we sneeze and get a bad endings every corner, rather make sure the endings are actually closer to the end of the game and have a realistic reason for it being a bad ending unless we decide to make a choice so stupid it would lead to something bad happening early on.
I feel like if you also do decide to add bad endings, adding multiple good endings in the future could also make the game more fun, especially if you wanna add an ending tracker in the game that players can use to try and find all the endings good or bad. heck maybe if we get all the good and bad endings you could add one final "true ending" to the game to add a neat little bow and tie up any loose ends but regardless keep up the work and im excited to see where you take this game in the future!