Depends on what you want to accomplish on the back legs. If you want the support of SMBs, then you can use them. But if you have a horse that hits the insides of his legs, I have used something like this: VenTech Slide-Tec Skid Boot Professional Choice (Equine - Supplies Tack - Sports Medicine - Splint
I don't use rear boots at all unless I'm going to be making a lot of runs. At big app shows, you might make as many as five runs within two hours. When horses start to get tired, they tend to hit their back legs.
Hope this helps.