I don't know if this is one of the 'rules' but I leave my butter out all the time, 24/7 regardless of the temperature.
I keep it in a china covered butter dish. It has never gone rancid and never melted. I use quality butter from Costco or the nearby grocer or the imported Kerrygold. Never had a problem and have been doing this for many, many years.
ETA: I remember a young woman I worked with some years ago who lived with her boyfriend in a trailer. She left the butter out on a dish on the counter overnight and when she went into the kitchen in the morning the butter dish was empty and surround by a circle of mouse turds.
