when is the Oubliette not worth it?

i always figured that i should never skip the level because more money and more items, but the cost is massive: 2 keys

and since that level only has 2 chests anyway, why not just use them to open up the ones in the first chamber?
also, the Oubliette is almost as difficult as the Hollow with all the massive rooms full of sporgres and vetern shotgun kin so you could end up pretty baterred when reaching Gungeon Proper even if the blobulon general was easy - because it doesn't give a master round

so should the level be entered only if there's an overabundance of keys? or should be entered no matter what even at the cost of green/blue chests in order to reach the Abbey and/or make enough money for the rat key?