Hard to fill in your poll..As my imps are all over the place. If I need the training speedier I put imps there. If I want stone from raids they go in the Dark Library. if I am away from my dungeon they go in the Torture Chamber. Infact I've used them everywhere. Currently I've got them all upgrading, so none spare to bolster...I actually want more imps, never thought I'd say that. But using them to bolster is great.