I too have used the General Finishes version of milk paint. I have followed up with GF High Performance clear satin water based poly. Usually give it about 3 sprayed coats of topcoat and seems to hold up well.
I think the answer to your question depends on your application. Is this Kitchen/Bath or is it something not subject to much wear/tear? GF has recommendations on when to use a topcoat and when you should use more than a satin poly. The GF milk paint is fine on its own. Their satin poly works well as a topcoat, adding some protection without changing the sheen much, like Wayne suggested. I don’t have experience with the other topcoats over GF milk paint.
I've used the real deal and have left it as is and also topcoated with a danish oil as seen on my pewter cupboard. It gave me the look I wanted. Even the as is has lasted over 25 years and looks just like the day I applied it.