If the windows are ajar for a long time, you should remember to close the radiator valves. If you left them in their regular position, the valves would be more open for that time and the radiator would become hotter, while the heat would be escaping through the tilted window anyway. This would be an unnecessary waste of energy.
Due to the way the thermostatic valves work, you should ventilate rooms briefly but regularly during the heating season.