I consulted Ann Kanable's excellent book
Raising Rabbits on the issue of salt for rabbits. She believes that some salt is important, epecially for rabbits on pellet-less or low-pellet diets. She thinks it is especially important for nursing does. Salt encourages the rabbit to drink water. With increased water consumption, the rabbit's appetite is better and therefore she is in a better position to create an ample milk supply. If you are using salt from the kitchen, make sure it is uniodized. Pickling/canning salt is suggested if the feed store spools or blocks are not being used.
Kanable goes so far as to suggest that does without free-choice salt may have youngsters who take considerably longer to reach butchering size. This may account, at least in part, for some of the disappointing results that people using rabbit tractors as grow-out pens have experienced.
What she says makes sense to me. I like this book very much as it is geared towards the needs of backyard or homestead rabbits rather than commerical ventures. I think it is time I reread it from start to finish.
Edited to add: I anyone is looking for a low-priced copy, try abebooks. They often have copies for about a dollar, plus shipping. Be sure to check the shipping rates... some are very reasonable, some outrageuous.
http://www.abebooks.com/