Should I keep cellular roaming on all the time?

Hi,

Data Roaming is actually made for traveling outside your country.

If you’re in your country, enabling data roaming or not won’t change anything for coverage.

When you’re not in your country, it also doesn’t change your cellular plan coverage - it just activates your cellular data abroad.

Learn more here: About cellular data roaming options for your iPhone and iPad - Apple Support

Remi_202DC has the right of it. Most of the time it doesn’t matter. Where it matters is if you are traveling outside of your country. And another, less obvious reason to turn it off is if you travel near an international border but within your country, where it might pick up towers across the border. US examples, if you don’t have Canada or Mexico coverage in your plan: Southern Arizona, Texas or California near the border with Mexico, Washington State, near the Canadian border, etc.