Jonathan Carr, the designer of Lyra cartridges says much the same thing as Atmasphere--loading is only needed with some phonostage which overload when presented with RFI.
I have found that with most high quality phonostages, not much more loading than the typical, built-in 47k ohm loading is needed. I once helped with a setup involving a Hovland preamp. There was obvious RFI interference (noise) even when the tonearm was sitting on its rest and not playing (external RFI). It turned out that the "default" loading on the Hovland was 100k and that just switching to 47k cured the problem.
The problem with a simple rule like 10X is that it will simply be wrong under certain circumstances. I have a Transfiguration Orpheus cartridge which has an extremely low source impedance of 1 ohm; the 10X rule would mean an extreme amount of loading at 10 ohms. Because it is a quite low output cartridge, that amount of loading would throw away a significant amount of signal and would mean, in my system, not enough gain. Also, a lot of loading does affect tonal balance--it substantially reduces the top end; I paid a lot of money for the kind of open and extended top end that good MC deliver and that would go to waste with this kind of loading. This rule is also not very useful when it is applied in the case of using a step up transformer where one has to consider where the loading is applied--on the primary side of the transformer or the secondary side?
The best approach is to try different loading to see what sounds best. If one is not so inclined, stick with a modest amount of loading that would work reasonably well under most circumstances, something in the range of 100-150 ohms--that would cure any RFI issues and is not so much loading that high frequencies would be severely attenuated.
