Short:
47 kohm when you have a preamp which can do it without sounding thin, dead and analytical 1kohm when you have a normal preamp below 200ohms when you have a very analytical and harsh sounding high end System and you want to soften it. or MM Phono and a matching high quality Step Up.
This short guide defines my experience to a tee: Jasmine LP2.0SE > DV10x5 > Rega P3.
The Jasmine has 4 settings (30ohms; 100ohms; 250ohms; 1Kohms). At the suggested cart load for the DV10x5 (1K) it's way to bright, highs distort and bass is muddy - in my current system. Same but to a lesser degree at 250ohms. 100ohms works best in my system. Well below the recommended load impedance but with the best over all results.