Well, pellets don't really keep the teeth ground down. They get crushed by the teeth, but rabbit teeth are made for chewing tough, fibrous plants like hay. The best diet for a rabbit's teeth is to include a lot of what they would eat in the wild- grass (hay or fresh grass) and a mix of veggies of different types and textures. Pellets should be limited both to prevent over eating and to encourage the bun to eat more hay instead. Some good chew toys can also be helpful for the front teeth, but more so for play as it seems that properly-aligned front teeth should wear each other down.
The perfect diet still won't guarantee that your rabbit will never have dental problems. If the rabbit's teeth aren't aligned right (genetically or through accidents or health problems), the teeth will eventually wear unevenly. So they should be checked regularly by your vet. The molars can't be checked with the naked eye- the vet should use a scope or lens of some sort to look at them.
