Quoted:
If your wearing plates always square to the target when ever possible. A squared stance allows you to have a wider field of coverage vs a assoslets/moddified assoslete stance.

Quoted:
(Edited to add some photos from the inter-web)
       
The best stance is the one which allows you best mobility, best ability to control and drive the rifle, and best protection if you are wearing body armor. It should also be somewhat comfortable! You shouldn't feel strained standing in this position.

The toes square to target, stock collapsed all the way in, elbows glued to your rib cage stance is from the days when subguns were all the rage and gas masks had limited visibility so people would sometimes put them in the middle of their chest to essentially point shoot. IMHO it does not transfer well to a carbine (actually I don't like it for subguns either). While you maximize armor coverage, your mobility is so-so, but your recoil control and ability to drive the gun from target to target is poor. The guy below has no leverage for moving the gun side to side, up or down with that magwell grip.http://www.everycitizenasoldier.org/sitebuildercontent/sitebuilderpictures/positions/.pond/S_UFP.jpg.w300h400.jpg



The fully bladed stance is good for competitive (highpower/bullseye) shooting, or very long range when you need to maximize your stability but are not concerned about mobility or armor protection. I will occasionally use this if I am shooting my sniper rifle, offhand, with a sling, and I have to make an accurate shot - but I'm not making quick follow up shots, and my ability to track a target is diminished. Supporting a heavy rifle for more than a few seconds is tough without some structural support - which this stance will give you.
http://www.jimstonefreelance.com/gun3.jpg



I think the best compromise is in-between. A good "fighting" stance, like you were going to box with someone. Slightly bladed, weight forward on the balls of your foot, arm extended further forward on the forend (NOT gripping the magwell). I like the support arm up a little bit (not chicken winging hanging out there to smack into stuff, but higher up on the gun, with the support hand wrapped around or at least on the side of the forend (not holding the VFG like a popsicle). With a standard M4 stock fully extended or maybe one notch in. You get pretty good protection, you have pretty good mobility, and your ability to manage recoil and "drive" the gun is maximized. I shoot this way for pretty much all my systems - carbine, pistol, shotgun - with the exception of maybe the sniper rifle. I would also add, your body size & shape matters. I am tall, thin with really long arms, so I may blade a bit more or reach farther forward with my support hand than others with shorter arms and stockier bodies.  
http://i428.photobucket.com/albums/qq10/tacticalyellowvisor/20120121%20Defoor%20Carbine/DSCN0031.jpg


 

Quoted:
Mostly square with dominant foot slightly back.

Quoted:
If your wearing plates always square to the target when ever possible. A squared stance allows you to have a wider field of coverage vs a assoslets/moddified assoslete stance.

Quoted:
https://www.youtube.com/watch?v=Wm6LGovgtrI&feature=youtu.be

Quoted:  Can anyone explain why, with so many SBRs out now, people are resting their faces up on the charging handle when they fire? Mr GunsNGear does this a lot.