Is it only necessary to do this the one time initially?

Yes, just do it with new cases or with cases that have been trimmed to length.
Peter.

Yes, you should only have to chamfer the case mouth the first time you use the case.

You MAY have to do it again if the brass flows and the necks thicken, but this would be unusual.

Of course, at some point you will have to trim the cases and then you will need to chamfer the mouths inside and out.