When to delete neighbors?

I tend not to delete them very often, but events are useful for figuring out who isn't playing. Sometimes near the end of an event, I'll go through and delete the ones that haven't acquired any currency throughout the entire thing.

But some friends I have kept around out of nostalgia, either because we helped each other a lot in past events, or they have such a well-designed town I still like to go and visit it.