To everyone starting Rebirth drama, the first thing that pops into mind is if there is a need to watch Princess agents. The answer i have gathered from a dozen weibo pages, reddit posts and most importantly the trailers and promotion materials released so far is, NO. 

The production team of rebirth and PA are two different companies, it doesn't make sense they will use PA as prequel to Rebirth as it brings legal and whole lot of problems.  Moreover Rebirth production team has bought copyrights of the novel and the author is part of production which was not case of PA. 

IN CONCLUSION,  YOU NEED NOT WATCH PRINCESS AGENTS TO FOLLOW OR UNDERSTAND REBIRTH. 

Edit

The first two ep have explained all events leading to the ending clif hanger scence in princess agents. More over, We have plenty of flashbacks through the starting eps to understand the dynamics.

Finally if you can sit through 58 ep dramas and watch, it's great, go for it. I can't watch anything before 2023, the rich colors of new dramas spoiled me.