Generally, if you're loading at X ohms and its sounds a bit dull/lifeless/bass heavy that way, try higher loading to get it to even out a bit. Alternatively, if you're loading at Y ohms and its sounds a bit too sparkly/tinny/HF heavy, then try lower loading. to get it to even out a bit.
That's correct. Most cartridge loading pertaining to sonics will be very subtle. The more resolving the system, the more you will hear it, but it will not change the overall sound aspect of the cartridge.
There are some that prefer loading a MC cart at 47K ohms, just like a MM cart. I have owned a few MC carts that do indeed sound better up that high.
The end game is: do whatever sounds the best to you. Again, you can NOT hurt anything doing this.