I think it is mostly a matter of taste in this case. I don't much like salt, so I will use unsalted butter. If I have to use salted butter, I will cut down on the amount I use. But that is just my personal taste. The wife, she will use salted butter and also add in the amount of salt it calls for. Heck, sometimes I think she adds just a little extra for good measure, LOL.
I just have never been a big fan of salt, but I can't remember my omitting it or cutting down on the amount used ever ruining a recipe.
And Andy is right, most butter here is sweet butter. European butter is very different, we had that discussion in a prior thread but I can't find the link right now.