In the 2004 holiday baking issue, food scientist Shirley Corriher explained that the ideal temperature for butter that’s to be creamed with sugar is 68°F or just slightly cooler. If it’s warmer, the butterfat starts to melt, leading to a collapse of the air bubbles you’re trying to create by creaming. So if you keep your house warmer than 68°F, you really want butter that’s a few degrees cooler than room temperature—it should be pliable but still slightly firm, not soft and squishy.
For her slice-and-bake cookies, Carole Walter calls for butter to be “slightly softened at room temperature” because she wants to limit the aeration of the butter. In this case, let the butter soften until it makes a slight impression when pressed with a fingertip, but still feels fairly firm. The temperature should be about 58°F.