Larry
Like many kitchen mantras this common advice is partially good practice and
partially rote repeating of "kitchen lore".
The basic reasoning is simple.
If a recipe uses a high ratio of butter to flour, Brioche being the obvious
example, then, if you are going to keep the salt in the correct proportion
in the final dough, you must either know the salt content of the butter you
are using so that you can calculate the amount added in the butter, or use
unsalted so that added salt can be measured accurately. The latter method
is often easier particularly since older recipes were codified before
nutritional labelling was mandatory.
That said, if a recipe calls for, say, a couple of tablespoons to a kilo of
flour (ca 3%) then the amount of salt added via the butter is smaller than
the accuracy limits of domestic measuring methods so it doesn't matter if
the butter is salted or unsalted.
In practice, most salted butters run at about 1.5 % or so meaning that, to
add a teaspoon of salt in the butter you would need to use 330 gm (about 12
oz).
If your smallest reasonable measure is 1/4 tsp then to add that much salt
in butter you would need 4 oz or so.
So, is it best to use unsalted butter for baking? As usual, the answer is
"sometimes".
There are very few absolutes in baking bread.
Hope this is of use
John