My room has "zero" slap echo. Do I need room treatments?

Probably; yes. I would have to think any non-treated room can be improved with treatments.

My room had no ’slap echo’ either, but in reality, it was a bit ’dead’, lifeless, and also built-up too much localized bass pressure. I didn’t think I needed any for the same reason until I tried experimenting.

In my case, I did find strategic dampening helped, but got my most positive results with diffusion, which helped break-up the sound and provide better imaging and soundstage. Your room may be different.

Suggestions? Experiment inexpensivly (foam or dense insulated sheets/panels) before spending more on specifically made acoustical treatments. Or, take what you used for experimentation and ’dress them up’. Though, this is easier to do with absorption than diffusion.

Slap echo is one hing and early reflections are another. You still want sound deadening at the early reflection points on the walls next to the speakers and the walls behind the speakers. Find those points using the mirror method. 

I agree, I had no echo either. I found tall silk plants/trees and some open shelving for my vinyl placed at first reflection points to bring everything into place. I have open timber running across the ceiling which is great for helping with diffusion and de fraction.

Thank you for your suggestions.  My ceiling has wood beams as well and is probably helping as you suggested and I have a tall silk plant (8') in the front wall corner.  Hope it is helping as well.  Thanks tom6897.

The left wall is all glass with full length vertical blinds.  I have put a tall backed upholstered chair at the reflection point.  Thanks mijostyn.

I am going to try some diffusion behind the speakers on the front wall.  Should they be placed in same manner as you would place absorbers?  IE where the reflection point would be.  Thanks bkeske