Rabbits do not lick for salt. Communal grooming is essential to rabbit society. Rabbits groom others in accordance to their social standing in the group. The alpha grooms no one, as my one lower is expected to groom others in order to be groomed by others. When you pet the rabbit they see that as grooming.
Though in some groups/pairs this is how it works out to be, I've found that grooming hierarchy is generally a different and more complicated process than just who's in charge getting the most grooming and giving out the least.
While it may be thought that it's just an alpha bun thing that controls the pecking order of communal grooming, I haven't found that to be entirely consistent. For some rabbits it will be that way, the bossy bun gets the most grooming from the subordinate buns, but I have a family group of rabbits that throw that concept to the wind.
The Alpha bun in that group used to be one of the brothers/son before he died a couple years ago, and he was pretty much a compulsive groomer of his siblings and mom in the group. It's almost like he demanded that the others let him groom them whenever and however much he wanted to, which of course none of them seemed too unhappy about
He would hop up to them and kind of body block them and start licking, and not let them leave til he was done. Then his sister and mom who are also bossy but just below him in being in charge, also compulsively groom the rest of the group, including the most subordinate buns.
But the the sister and brother at the very bottom of the pecking order in the bun hierarchy, are the least likely to groom the others. They will groom on occasion, but not as much, and the one at the very bottom rarely does. In fact it took her a long time to get to the point where she would reciprocate grooming at all. Those two are always the ones that get chased away if the boss buns want them to move, but yet they rarely groom the others and always get groomed by the boss bunnies, which they love and don't mind at all.
The group now has a different boss bunny, from a pair I bonded in, and the female of that pair is now the boss of the group. When it was just her bun companion as a pair, she was the boss but yet did more of the mutual grooming than him. Now as a group, she will demand to be groomed, but also reciprocates quite freely with those below her. And the two that were at the bottom of the group hierarchy before are still, and still the least likely to do any grooming. So when it comes to a hierarchy with grooming each other, it seems to be completely different than the pecking order hierarchy.
I also do feel that some rabbits do like to lick our skin because of the salty taste. Maybe not primarily because of the salt, but in conjunction with wanting to do the mutual grooming thing, will just lick more because of our salty skin.