Every time. Although there are times at night where I don't flush (pee), because I fear waking up the demented woman in the apartment below me (if I couldn't hear her for some time). She likes to rumble around at night. Sound insulation doesn't exist in my place.
Related question: Does anyone close the toilet lid before flushing your poo or do you not mind E.coli on your toothbrush? Whenever I read about it, I plan to do it, but I forget it everytime. Hard to kick the not closing habit.