There's a script I want to run on the Journal Entry record; I'd like it to check the account type for all lines and, if they are "Bank" accounts, to set a custom field in the header. Is that a Validate Line function or a Field Changed function (or both)?

If you only care about this logic being applied in the UI, then I would probably go with validateLine. The scenario sounds like a better fit for a user event script rather than a client script though to make sure it works on JEs that are not created manually in the UI (e.g., CSV Import).

You can do this in the saveRecord client event so that the user is done adding lines before you set the custom header field