Gigi (or anyone) - what is considered best practice when it comes to selecting relays?

This is something that I don't often see mentioned but feels like a critical yet unintuitive element of the Nostr user experience.

Some questions to illustrate the point:

- How important is it to select relays that are close geographically?

- How many relays is optimal and is there a performance trade-off from having too many?

- Are paid relays deemed necessary at this point in time for a new user?

- Do Nostr clients have the ability to change which relays you use without specific user intervention, or are relays entirely client-agnostic?

Cheers.