I think that adding fascia depends on what method you are using for building your railway.
If you are using the cardboard/ plaster cloth method, the fascia (so I'm told) is added right after completing the bench work so that it can be used to support the cardboard strips and plaster cloth.
If you are using the foam and plaster cloth method (which I like to use) I find it best to add the fascia after the foam has been carved so that I can shape the fascia to the landscape without having to go around trees and track and breaking things by accident.
If you are doing a basic table top layout without much landscape to it (in other words it is flat) the fascia I guess can be installed any time.
A mistake I had made repeatedly is that I forgot about the fascia until after the layout was scenically "finished". Working around everything that you have already built can be a hassle. Thankfully This time around I have gotten it right
hopefully.
In conclusion, the best time to add the fascia is when it is the most oppritune moment. Adding it when finished, and cutting it before the layout bench work has even been erected causes problems.