Does the reader mention what part of the country she lives in? I live in the northeast (NY.) This time of year it does save me money to use 1 electric heater in the main area of the house- it keeps the central heat from starting up. The weather's been 50's daytime, 30's night. Bedrooms get quite cool, but the core of the house stays warm. If it got no less that 30 degrees, we could probably heat with electric.
Once we get into the 20's and below at night, The electric heater isn't enough. At that point we let the central heat (oil) kick on. Leaving the 1 electric heater on almost all the time costs about $50 a month (cheaper than the oil would cost.) Heating with multiple electric heaters throughout the house would be very expensive.
Another factor to consider; on the second floor, if the first floor is occupied, how much does their heat warm the upstairs? Could be a lot.
Edited to add: our house has large windows and a little sunshine heats it up during the day.