Breast collars are a Great thing to have if your trailriding! It helps keep the saddle in place when going up and down hills it also helps to keep it in place when the girth gets a little loose. I know some people that wont even think of riding with out them. And if you get a synthetic one they are very easy to clean
I use a Mohair breast collar.. Frassy loves it! The mohair wicks the moisture better than leather or biothine.. Just like her mohair cincha. I agree that the use of a breast collar helps keep the saddle in place and lets you use less pressure on the girth area. Also I recomend the use of a crupper. When the horse gets accustomed to them they use them as tools when mounting, going up or down hills, and pulling downed tree limbs. When I mount or go down a hill I can feel Fras clamp down on th crupper with her tail muscle (one of their strongest muscles) to keep everything in place. I use them every ride, even short ones so the horse toughens the hide under the tail and learns to use it.
Here is Frass collar
I used to use one a lot with my old saddle which fit well on it's own. I was borrowing a Crates saddle that fit Cope pretty well, but for some reason it caused the chest collar to rub the spot where the three pieces attach to each other in his chest. It made a little bald spot there, so I took it off. I feel kinda bad because I've been looking for one to work with the Simco saddle I have now, but haven't. We're going on an 18 mile trail ride tomorrow...so I guess it's a good thing the trail doesn't have many hills... There will be a tack vendor there though, and I plan on seeing if they have anything I like.