Is the rules event "Before saving a user account" applied every time a user edits their account? I would like to apply a rule that allows existing users whose email addresses come from a certain domain to continue using their existing accounts, but to prevent the creation of future accounts from these domains.

I have configured my rule to react on the After saving a new user account event, but I'm unsure as to whether to have it react also on Before saving a user account.

Does saving here refer to only to account creation, or also to account updates such as a password change?
Before saving a user account will react before you make a change in the user account.

And After saving a new user account will react After you create the user account.

But if you have the 2 events once you create a new user account you will trigger first the Before saving a user account event and after the After saving a new user account event.

If you go to create or update a user account always you need to call the user_save() function. The difference is in the first param. See this question with an example:

Create a user programmatically and give they a role