For butts, I salt fairly heavily - certainly not covered - let's say a 'generous' sprinkling. If you are unsure, err on the side of caution and go a bit lighter. One can always add more post cook. Once the pork is pulled, chopped, or whatever, much of the bark gets blended in (though I like bark-y nuggets that are left alone).
Sidenote: As noted here and in other threads, I always salt first, then apply the rub over the salt. For thinner cuts, like ribs, e.g., I salt as if the meat was served to me already cooked but unsalted, i.e., the amount I would normally use in that situation.