I - bIII - V (Chord construction)

I use theory to add to my compositional skills but I don't rely on it. If you write a song that's harmonic it's nice to know exactly where your thirds are etc when harmonizing. Knowing your progressions can help as well.
I often actually use theory to determine if what I'm writing is too similar to something else. I'll think about the progressions that I'm using and determine if I simply took another riff and transposed it. In cases like that I just scratch the song and start over or think of new ways to be creative while avoiding those progressions.
I kinda had to explain to my guitarist that we can write an albums worth of i-VI progressions. ATG already did that anyway.