Personally, I never ride a horse without one. For me, not only is it a training issue (I need them to be used to pretty much whatever type of tack they might eventually wear someday), but it's a safety/comfort issue too. A back cinch just keeps the saddle that much more secure on the horse's back. There is less movement in the tree and, therefore, less chance of the horse getting pinched or sored from the saddle rocking on their back while riding.
Another thing to consider is how the saddle is rigged. If it is a full or 7/8ths rigging, then you really should
use a back cinch for anything other than leisurely trail rides on flat ground. On those saddles, any sort of downhill work or work with quick speed changes or turns can cause the saddle to tip forward as the back of the saddle comes up. When that happens, the front of the bars dig into the horse's shoulders and can cause some real problems. If it has a 3/4 rigging, then the pressure on the tree is more balanced between front and back and you probably wouldn't need to use a back cinch.
You can see the differences between the riggings here. Western Saddle Rigging