You should check which vitamins rabbits need and make sure they are getting some foods which provide them, I wouldn't just assume that because you give them a varied diet all their vitamins will be provided.
Well I feel Annie is getting a good varied diet and she seems to be in top health so I'll stick to that unless there is a need to change I just got worried when a Miss-Know-It-All came to visit :roll:
Rabbits are evolved to live on a poor quality diet - they originated from places like the arid parts of Spain where there's little else other than parched grass! There is far more danger of giving them too much nutrient rather than too little. Science selective pellets nutrition information shows it has added vitamins and minerals anyway so if a few of those are being fed as part of a broader hay-based diet then I certainly wouldn't give any extra - it would be like doubling up on added vitamins.
No, it's just like humans - if you're eating a proper balanced diet then you don't need those kind of supplements, and they could do more harm than good. Wild bunnies spend all day munching on grass and whatever plants happen to be around and they don't need supplements.
In particular I've never understood salt blocks for bunnies :shock: