For best results, keep butter in a covered dish or butter crock and place it away from sunlight, heat, and humidity. Only leave out a small amount that you expect to use within a few days.
Unsalted, whipped, or flavored butter should usually be refrigerated because it can spoil faster. Butter should also go back in the fridge if the kitchen is especially warm.
If butter develops a sour smell, unusual color, strange taste, or mold, it should be discarded. So yes, leaving butter on the counter can be safe, but proper storage and temperature are important.
