What cables are you using for a hidden in wall clean look? I have B&W 802 D3s coming.

Is plenum rated that important for in wall speaker cables? 

@mgould This is one link: http://www.bluejeanscable.com/articles/inwallrating.htm

Here are a few quotes from that link:
"The main purpose of NEC is to prevent hazards to human health and safety from wiring, and these fall primarily in a few categories: electrical shock, tendency to start or perpetuate a fire, and production of toxic fumes when exposed to fire."

"To classify the types of cables suitable for reducing these risks in various applications, the NEC provides a system of ratings of cables. An example would be CL3, or "Class 3," which many people recognize as an "in-wall" rating because of the labeling on the spools of speaker wire found at home improvement stores."

"A "plenum," under Article 100 of NEC, is "a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system." The most common "plenum" space one sees in a/v installations is a dropped ceiling space in a commercial building, being used as a ventilation return. Most residences do not have any significant plenum spaces, so this is rarely a consideration in a residential installation--but on occasion it can be. Plenum cables are required to have jackets and dielectrics which don't easily give off toxic fumes when burned--the reason being that a fire in one part of the building can, through the ventilation system, feed toxic fumes to the entire building."

...It is a good article if you are detail oriented.  And, it's always good to understand what is involved when you are "breaking the rules" (as in the electrical code etc). I have previously run speaker cables under the floor from an amp to speakers, but that was a long time ago in terms of where my knowledge is today. For a casual space, I'd run cables behind the scenes. For a critical listening set up: No way, out in the open and away from any other cables etc.

We used bulk #12 gauge stranded copper cable throughout most of the rooms on the first floor while our house was being built. We had to use "code" cables otherwise the house would have not passed the inspection for occupancy permit. I also put speaker terminals in certain spots on the walls where I expected the speakers/amplifiers to be placed. The "code" cables are somewhat rigid and are not as flexible as "normal" speaker cables but I figured a #12 gauge copper cable would do the job. I even put speaker terminals in a few places way up on the walls where I thought I might put surround speakers. Right now, I’m only using some of the terminals for a 2-channel set up. Bottom line is, make sure you check your local safety electrical code regulations just to avoid issues down the line, for example when you decide to sell the house, etc. As an FYI, you can look all this up by a trip to your local Home Depot (if in US). They even sell cables down to #2 gauge which look quite impressive....but there’s no way you can connect anything less than #10 gauge to any off-the-shelf connectors. Those #2 gauge wires look awesome by the way...