From someone who normally closes my flightplan in the air:
I usually ask to have it closed when I'm within gliding distance of the field, there is almost always someone waiting for me or for other reasons are around who will witness any mishaps.
I have forgotten to close it once and that resulted in a very unpleasant phone call with warnings of potential heavy fines if it happened again, result: I am a lot more scared of missing to close it than I am of being alone in a field somewhere in a crashed AC