Bass is kinda "everywhere"....and not as localized and directional as mids and highs....so, put the bass trapping where you notice the most bass build up....usually corners and wall/ceiling points.
Play some commercial music with strong bass content...then walk around the room, and take not of where the bass gets noticeably stronger and where it seems to drop out....those are your nodes and nulls where bass trapping can help even things out.