The seat must be level for two reasons. First, for the rider. The rider will be much more secure, regardless of depth of seat, knee rolls or pommel swell if he is sitting balanced. Centered Riding instructors do a great demonstration of how powerfully your balance contributes toward sticking in the saddle. Not only is the rider more secure, but riding takes much less effort if the bones of the skeleton are doing the “work” rather than muscles, which is only possible sitting balanced. And if the rider does not have to work to stay centered, the horse does not have to work to overcome shifts in balance at every stride.
The second reason also helps the horse. If the seat is not balanced for the rider, then the rider’s weight is possibly driving the saddle in the direction that caused the unbalance. In other words, if the saddle is tipped forward because the horse has a downhill conformation, then rider is likely pushing the saddle forward as well. If the horse is uphill, the rider may be landing hard on the cantle and if the saddle shifts back, then the rider is possibly behind the horse’s weight carrying area.
There are other reasons that the saddle can be out of balance for the rider. If the seat is too small, the rider’s seat bones may be back up on the cantle, tipping him forward. If the seat is too big, the rider may be forward of the center of the saddle and tipped back. Improperly positioned stirrup bars can create or exacerbate this situation. Sadly very few saddles are designed for correct riding in this aspect.
The rider must determine what works for him and his horse as a package.
Some saddles are designed for men and some for women. Unfortunately, we find that most are designed for men, even though most riders are women. Typically, a woman’s pelvis is deeper than a man’s. Her seat bones will be positioned further back and a bit wider on the seat. If a saddle was designed to sit level on a particular horse and made for a man, then a woman may feel tipped forward and vice versa.
A woman will also generally have a smaller foot, but since many saddles were designed for men, the stirrup position will be too far forward. This causes women to be constantly searching for the stirrup, forcing it back into a proper position. In actuality, stirrups are often placed too far forward for men as well. There are two major problems with this.
First, it is not comfortable and stresses the legs and back while fighting for position. Posting becomes an effort when it does not need to be.
Second, it forces riders to assume the chair position, which is good for neither the horse or rider. If your legs are out in front, you are forced to push against the stirrups, which pushes your rear end up the cantle. Many people think they need a bigger seat than they really do because of this.
The first check for a level seat is visual. The horse should be standing square on level ground. If it looks unbalanced, it likely is. But eyes are deceiving because you may be used to one type of saddle and looking at another. An english saddle will look tipped forward to one used to a western saddle with a large pommel swell and horn. It will probably feel that way in the beginning too!
In any case, the best judge for determining if the seat is level for YOU is to go for a ride. How does it feel? If you can have an instructor or other knowledgeable person look at your position, that's great. Pictures can be helpful if you want to judge for yourself later.
An observation I've made over the years: A new rider will do best if everything is perfect (obviously) and if not, they may feel uncomfortable but not know what's wrong. An intermediate rider (and for this I include all but beginners and very high level riders) will adjust and not even realize something is wrong. They're strong enough to overcome imbalances, possibly later realizing they've been putting up with something crazy like different length stirrups or a tipped seat. Truly advanced riders are often able to realize something is off and exactly what it is. The rest of us will strive for this!