It depends on what you're doing and who you are. If you want to maximize privacy:
  • segregate identities (npub/nsec) based what you're using them for
  • if you don't need a persistent identity for something, use an ephemeral one
Most mobile clients only support a single identity, so I tend use them with my main identity. For anything where I need a segregated id, I use it on the web with Alby.