mc cartridge loading

Not worth the effort if you are talking about the difference between 100R and 90R.  However, when the manufacturer says "at least", that implies that higher resistances (less of a load on the cartridge) might sound even better than 100R. Whether you would think so, if you tried it, depends upon your personal taste, the reactance of your phono cable and phono stage input, even the topology of your phono stage.  MC phono cartridges are typically loaded down in order to tame a rise in response at very high frequencies that can occur when the aforementioned factors are in a certain alignment.  But in some cases, MC cartridges can sound surprisingly great into a standard 47K load resistance, a la that used for MM cartridges.

to lewm
Thanks for your response.
I might try reverting to 200 ohms, which is what is pre-installed in the phono amp, so involves removing the resistor I added in parallel without replacing it.

Right now, on two different turntables, I have set up a Koetsu Urushi, in one case, and an Ortofon MC7500, in the other, both looking at a 47K load.  Sounds really good, but not necessarily better than into, say, 1000R or even 100R.  (I do think I like them better into 1000R or 47K vs into 100R but would need more listening time to be sure.)  This is using an Atma-sphere MP1 preamplifier.