It’s perfectly safe to leave salted and unsalted butter out on the counter, if the kitchen is cool. Past 68F, butter will soften, and the texture will suffer.
Adds Robert Lauriston, “Spoilage shouldn’t be an issue if you eat it regularly and leave only a quarter pound at a time at room temperature.”
Allstonian uses a Butter Bell in warmish weather to keep the butter fresh longer. The container keeps the butter over a well of cold water, which keeps the butter cool but still spreadable.
See a selction of these butter keepers at Amazon.
salted butter better to leave out