Whether or not you should put boots on all four legs of your horse depends on a few factors besides your personal preference. If your horse tends to touch the poles with its hind legs, it may make a lot of sense to protect it with tendon boots. As tendon boots also have a support function for the tendon, they can also be of good service if your horse is susceptible to injury.
You should always protect the front legs when jumping, not only because of contact with the poles, but also to avoid injuries from kicks from the hind hooves. Basically, it is better to protect all four legs with tendon boots.