As my sig indicates, my favorite wines (for now, anyhow

I know that a wine like a Brunello needs to be decanted, but my question is (other than if there is clearly sediment in it) when should a wine be decanted? Are there certain 'guidelines' for what types/age/price/etc of bottles should spend some time breathing?
Generally - and I'm not saying I'm right here, just how I've been doing it - I have been decanting most anything that I knew was real strong (like a Brunello or Rosso), or had been aged a bit, or I had paid a pretty penny for. However, recently I have had some wines that either were not very expensive, or were not the types I thought you 'normally' would decant and so I didn't, yet when I drank them I found their flavor was going through fantastic changes after spending some time in the glass and/or the bottle being open! For example, just the other day I had a Marquis Phillips Sarah's Blend (not more than $18 or so) and not a wine I would have thought to decant... yet, after spending some time in the glass, and even the next day when i finished it, I felt it was a lot different and surprisingly a lot better than it had been fresh. The thought hit me, "wow, maybe I should have decanted that a bit!".
And so, I thought I'd ask you all your thoughts on the topic. Other than the obvious ones, generally what types and what wines (price? varietal? age?) should spend some time breathing? And when is it clearly not needed, or even bad to?
I'm sure there isn't a 100% blanket answer that covers everything, as no generalization holds true across the board, but what're your thoughts on this? I'd love to know.

Thanks!!