Yes, generally. Covered butter can sit at room temperature overnight, especially if it's salted, but a hot kitchen changes the math.
Butter behaves differently from most dairy because of what it's mostly made of: fat, not water. Bacteria need moisture to grow, and butter simply doesn't offer much of it.
That's why leaving out a stick for easy spreading is common practice in a lot of kitchens, while doing the same with milk would be a bad idea.
Butter is roughly 80% fat and only about 16-18% water, and that low moisture content makes it far more resistant to bacterial growth than milk, cream, or soft cheese. Salted butter has an extra edge, since salt acts as a mild preservative and further lowers the water activity bacteria need to grow.
Covered, salted butter kept away from heat and sunlight is generally fine at room temperature for a day or two, and many people safely keep a small amount out for a week or more in a butter dish, topping it up as they go. Overnight is well within that safe range for most kitchens, and this is common practice in plenty of households and even some professional kitchens.
Unsalted butter has slightly less protection and spoils a bit faster, so refrigerating it is the safer default if you won't use it soon. In a hot or humid kitchen, or during summer without air conditioning, refrigeration is also the smarter call regardless of salt content — butter left in real heat can turn rancid, develop an off flavor, or even melt into a greasy mess on the counter.
Rancidity shows up as a sour, slightly cheesy smell and a yellowed, waxy look at the edges. It's a quality problem more than a dangerous one, but once you notice it, the butter is better used in baking than spread plain, or tossed if the smell is strong.
Margarine and whipped butter blends often contain more water and added oils than pure butter, which can make them slightly less shelf-stable at room temperature. Check the label — many spreadable tub blends are actually formulated for the fridge, not the counter.
Overnight on the counter is not a food safety gamble for covered, salted butter in a normal kitchen. Just keep it out of the heat, and refrigerate unsalted butter if you're not using it right away.
Not strictly, if it's salted, covered, and used within a week or so. Unsalted butter and hot climates are the two situations where refrigeration is the safer choice.
Salted butter, covered and away from heat, is generally fine for up to a week or two at cool room temperature. Rancidity, not bacteria, is the main long-term risk.
It largely comes down to kitchen temperature and habit — both salt content and typical room temperature affect how long butter stays safe unrefrigerated.
Yes — rancid butter smells sour or musty and can taste off or slightly cheesy. Discard it if either happens, even though it isn't usually dangerous spoilage.