As pavelski says, there is no set rule for stance width and for me depends on conditions and need.
Racers will often adopt a seemingly wide stance and this makes sense, in that they are looking to increase stability and reduce risk often on icey and rutted courses. I prefer a medium stance for free carving, but this is really only in the transitions. Once your legs start to incline beyond a certain point, the skis must move farther apart. How far the skis move apart depends on how extreme you tilt them.
It takes a while to get the inside ski out from underneath you and trust your edges (two of them). At some point you may discover boot out from the inside boot once it starts getting out there.
Angulation gives stability.To start angulating you will need to adopt a little hip counter. This means that the hips face a little toward the outside of the turn. (Interestingly in that extreme carving snowboard video, you will see little angualtion)
Most skiers, even those that carve well, will only carve a turn through 60 to 90 degrees. The real joy of carving is taking the turn beyond that and railing through 120 to 180 degrees. The forces are greater and so is the thrill.
Most skiers talk about pressuring the ski. What beomes more important is how much downward pressure you can apply to the edge/s. This is an equal and opposite reaction physics thingy. You have a big mass, your body and to lift it up a little, you have to press downwards on the ski edges. This is a downward pressure, not an outward pressure. Here's the weird part, the lower your body is, the more pressure you can apply and the more your edges grip. You will also need outward pressure on the ski (esp the front) to flex the ski and determine somewhat the turn shape. I like to think of these two presures seperately - downward and outward. What makes a lot of skiers skid the end of the turn is (in part) that they are standing so tall, that they can't maintain the downward pressure). Just your weight is not enough to do it at the end of a turn when the forces are greatest.
There are basically two ways to apply this downward pressure.
First is a "cross over" turn. Named because the body crosses over the skis during transition. In this turn the skier stands tall at transition, but sinks into the turn and rises out to transition again. The important point to understand is that "the sinking" is downward not outward. The sink must happen very fast - as soon as the skis are on the new edges.
The second way is a "cross under" turn. Here the skier has legs flexed at transition and actively extends the legs outwards and downwards as the skis roll onto the new edge. The body shouldn't rise through this extension of the legs.
In both turns, there should be little weight on the skis at transition and and the edge change should be fast.
I hope that makes some sense. Like so many skills, it's a matter of finding the trigger to understanding. If it's hard to find good models to copy, try resorts that have the most racing, which tend to have the most ski carvers off the race courses as well.
I'm loath to link to this video, as this was my first time out this season and shows it. Certainly not a model for the best technique. It does show my stance width and the difference between turns that are purely inclinated (due ot some lower back pain, fun, but less stable) and those with some hip counter/angulation.
http://video.google.com/videoplay?docid=-5521147838682881679&q=ski+carving+-+no+poles&hl=en
Trencher