Rigging and girth position.

The idea that you need to put the rigging forward on the saddle so it hangs vertical in the "girth groove" of the horse or it will pull the saddle out of position is a fallacy. The position of the saddle on the horse is determined by the shape of the tree matching the shape of the horse, not the rigging position or cinch placement.  

The cinch will move to the narrowest part of the chest (unless held away from it by something like a center fire rigging position).  But an angled latigo and cinch will not pull the saddle out of position if the tree fits.  It is only if the tree really doesn't fit the horse or there is excess padding which negates the fit of the tree on the horse that the rigging position can have an effect on saddle position.  Rod has used his 3/4 rigged saddle (pictured above) on many different horses over the past almost 25 years.  The only time the saddle has moved forward was on a couple really wide, round backed horses it was too narrow for.  On one "interesting" backed horse he used a lot, he actually needed a breast collar to keep it forward as it tended to slide back on that horse, even with the latigo angled forward.
It may also appear that a latigo and cinch that is angled forward would cause the saddle to put excess pressure on the back of the shoulder blades, while a saddle with a full rigging position (which is most likely to have a vertical cinch) would be least likely to affect the shoulders.  However, this is backwards from what has been shown by pressure testing (unpublished data) to happen in real life.  The more forward a rigging is set, the more it pulls down on the front of the tree.  The farther back it is set, the more it can pull down more evenly on the whole tree.  Of course, rigging design and construction has a large effect on how and where the rigging pulls down on the tree, but practically, a rigging set below the fork will pull down almost exclusively on the front of the bars.  Therefore, a forward rigging position can actually cause pressure points at the back of the shoulder blades because it pulls the saddle down tightly at the front and so it doesn't give the shoulder blades enough room to slip underneath the front bar tips so they "bang against the wall".  The same saddle with the rigging moved back could allow free shoulder movement.  A rigging that is set farther back will probably end up with a latigo that is angled forward, but it won't pull the saddle forward and it will free up the shoulders more than a forward set rigging.

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The Bare Basics Summarized

1.)  Saddle position is determined by the tree, ending up where shape of the tree matches the shape of the horse.
2.)  Western saddle trees are made to sit right behind the shoulder blade, which puts the skirts just over the back of the shoulder.
3.)  If the tree fits at all, the saddle will move to the correct position unless you keep it out of position with a breast collar, crupper or britching.
4.)  Rigging position does not determine or change saddle position unless the tree doesn't fit the horse or over padding causes the tree to no longer match the shape of the horse.
5.)  Rigging position will not move a good fitting tree out of position.
6.)  Forward rigging positions can impede shoulder movement.
7.)  The skirts need to be well blocked to follow the relief built into the bars so they don't interfere with the shoulder blade.