In modern cooking, cooks often prefer unsalted butter, giving more control over how much salt goes into a dish. Salted butter tends to be preferred when used as a table condiment for bread, muffins, etc.
In 19th century and earlier cookbooks, however, I have found no reference at all to salted vs. unsalted butter. What I have seen, is in cases where lard or butter are given as options, it is recommended to add salt if lard is used, to make up for the salt that would have been in the butter. I also note that may recipes that call for butter do not call for any salt. This, along with the fact that salt is a preservative, indicates to me that all butter of the time was salted.
So, by all means, use unsalted butter if you prefer. But keep in mind that you may need to add salt to the recipe to compensate.