Pretty much any room will benefit - especially in the bottom end. Yes - some depends on the room, furnishings, and speakers themselves. I will disagree that you can fix everything with EQ. You can help with peaks (not nulls) with EQ - but that's only in the frequency domain.
Treatments are required in many cases to deal with early reflections (depending on speaker type and directivity). In almos all cases, treatments are required to get the overall decay time of the room under control from 250Hz down. You can have the flattest response you want but if bass is ringing on for a full second or more, it's masking detail, muddying things up, and actually costing you bass extension and tightness.
Obviously, some will consider this a slanted point of view due to my business association. I'll just say try it. Go to the hardware store and grab a few bales of insulation. Stack them up in some of the corners of your room and listen for yourself. It's cheap and easy and you can return them when you're done. It's not an end all but can give you an idea of what you're missing.
Bryan