100 Ohm is standard, basic recommendation for many MC, also default for the most phono stages. But you can use whatever YOU like, can be 300, 500, 1000, 10000, 47000 Ohm.
It’s nice to have a phono stage with optional loading, not every user have an optional loading, normally no choice, except 100 Ohm (or very few alternative options). So when someone using 100 Ohm and ok with that, it doesn’t mean he/she tried many different options.
Do not hesitate to try different loading if you can, it will not make any harm for your MC cartridges.
It’s nice to have a phono stage with optional loading, not every user have an optional loading, normally no choice, except 100 Ohm (or very few alternative options). So when someone using 100 Ohm and ok with that, it doesn’t mean he/she tried many different options.
Do not hesitate to try different loading if you can, it will not make any harm for your MC cartridges.