Depends on how you want to experience it.
It can be fine to just play the base game story to it's point of no return, then head into Phantom Liberty.
You can also dip into Phantom Liberty early and do the first few missions until you regain access to the rest of the game (Lucrecia My Reflection, the 3rd mission) so you can access a few of the PL features like the iconics from Airdrops, car acquisition events, unique cyberware and Relic skills. Then continue playing base game until the point of no return.
Or you can simply head into Phantom Liberty right away and get to it's point of no return before going back to the base game.
Either way is fine. It just depends on your preference and how you want to experience the story.
Personally, I like to dip into PL for those few missions, just to access some of the items and relic skills since they provide additional gameplay options to utilize while playing the rest of the game.