There's no hard and fast rules with room tuning as there are just too many variables.
Loudspeaker performance and personal taste in presentation being the two biggest.
For example absorption at first reflection points may narrow the soundstage and if you want wide soundstage this will not be for you. But do the speakers have wide or narrow directivity? This will also affect that decision.
Personally I would not randomly build in any treatment to the construction as if it doesn't work out ripping it out will be a real pain. For a dedicated room where you can do what you like I would start by setting up with normal furnishing, do some extensive listening to adapt myself to the acoustic, and then consider treatments.
Hopefully there will be no low frequency problems so then dealing with the rest of the audio band will be a matter of taste. IME most situations will benefit from diffusion on the wall behind the speakers and absorption on the wall behind the listening position. But nothing about room treatment should be taken as gospel.