Snow plow, wedge, pizza slice... whatever this is called, you better forget about it. I have argued this long and hard all over - here is a collection of thoughts and ideas on the subject.
The only valid arguments for using the snow plow that I have ever encountered is in a very narrow corridor or the flats... And even there, only sometimes. Some also bring up the fact that it's hard in some crowded resorts to teach parallel skiing.
The bottom line is that the snow-plow is a bio-mechanically weak position, which is ineffective at slowing, stopping or steering. Much better alternatives exist when the skis are kept parallel.
Just because a ski, as opposed to a skate, can be used in a wedge, it doesn't mean it should, no more than a helmet should stay in a backpack... So, forget about the wedge, unless you don't like your ACLs, that is. Given the mechanics of a fall with the knees rotated inward: your ACL is in danger and maybe more.
Also, along the lines of debunking this position, you should read the good book The Physics of Skiing, the section on injuries, to discover how this position, taught all over the world, is probably responsible for the lion's share of torn ACLs and other knee injuries.
Women are especially susceptible to ACL injuries, so watch out!
A fall in a wedge is either forward, over the knees which are already in weak position, or backward, both bad. If you instead were stopping with the skis parallel, in a hockey stop, the most likely is your feet skidding away from you and many skiers usually recover from that without even stopping.
The snow-plow works only on flat terrain and very slow speeds. This is precisely why I think it should be skipped and not be taught at all: it will become ingrained, gives the beginners a false sense of security and they will use it on blue/blacks wherever and totally get in trouble. What's the point, when hockey stops work well everywhere, except maybe in a crowded lift line or some narrow alley? Once a beginner can do good hockey stops, he will safely pull off a snow plow wherever he needs one!
Certainly can't ski ice in control with a snowplow... the simple fact is that you can't tip the edges at an angle sufficient to grip effectively on ice, never on a black, not without blowing your knees out.
With a sideways skid / hockey stop, your bio-mechanics allows it quite easily: you can in fact angulate sufficiently to grip - read Ron Le Master's bible carefully, the section where he describes how the skis hold...
Read Ultimate Skiing++.
In fact, to grip on ice, you need to tip the skis at fairly high angles (to the slope, not the vertical) and put all weight on one ski only...
Also, because the snow-plow gives people a false sense of security, the snow plow is likely responsible for many other, more serious injuries and even deaths. A beginner's feeble and inherently weak snow plow stance cannot slow one down, stop him or even steer her around the tree or other kids crossing her path, once that beginner gets too much speed, unintentionally, of course.
Training books state simply that, in order to learn good habits, the bad habits must be erased and replaced it with the good habits. This negates the possibility of a progression from wedge to parallel. What one should do is simply stop keeping the skis in a wedge altogether and start with a direct parallel progression from scratch!
In terms of slowing or stopping, the hockey stop is what one must think and practice.
For steering and also controlling speed, any manner of turning with the skis parallel is much better.
Effective skiing is carving on the outside ski. Here is a progression, among many.
Just tip the skis on their left edges and balance on them. Keep them there and they will turn to the left. The edge may get caught only if you try to pivot them, possibly, i.e. force them to turn rather than letting them do the turning.
At low speeds, your body will be more or less between boots no matter what you do - there is no centripetal force to lean against. If the feet are tipped to the left, your upper body will necessarily angulate to the right, to keep you in balance. Nothing more, nothing less. Roller blades? Check! Angulation! Check!
Just keep angulating your body more and more, allowing the skis to tip more and more and thus turn more and more.
Next is braquage and pivot slips kind of thing culminating in a hockey stop. If you don't tell them "it's hard" or "watch out for skiis flat"... it won't be!
That's my recipe anyways... As usual, mileage may vary. And it's obviously adapted to the exact circumstances... But yeah, that's the general idea: get them to balance on their edges. It is important to start and go through the motions stationary, a few times... Before gliding the first time. I found that makes a big difference.
My favorite drill is just this: tuck roller blades turns with counter, on a green. You should hear good 18 year old racers grunt when directed to do that, yet i always make sure they do a few as often as possible.
And if it's beginners, they should always start with 10 minutes of skating at the bottom of the hill, to get the hang of balancing on the skis. I realize that the average lesson seeker wont have the discipline of a kid being yelled at, that's a challenge, no doubt