Proper temperature takes a back seat to proper relative humidity. When the temperature changes then so does the relative humidity. I would say as a rule of thumb that colder is better than warmer, because lower temperatures RAISE relative humidity, and higher temperatures LOWER relative humidity.
Big changes in temperature are not good for the instrument, particularly very low temperatures(below 40 degrees F for example) that are then changed to much higher temperatures. Let’s say for example that your harp is in the back of the car in winter, where the temperature is maybe 40 degrees. If the harp is left there long enough, say over night, or on a 3 or 4 hour drive, then the whole harp descends to 40 degrees. There is nothing wrong so far. But, if you then take your 40 degree harp into a room that is say 70 degrees F, condensation will form on the instrument, including inside the action. This could do serious damage to the finish, metal parts and heaven knows what else. So it’s probably a good idea to try to keep the harp at a consistent temperature.