From the perspective of someone who started the program in March, and just took their PAR written exam this morning (about 2 hours ago), I would recommend getting it out of the way prior to coming.
That said, do not stress about it if you are unable to complete them prior to your start date as many students complete them about a month or so into training. As you’ll see by reading through this forum, the ATP program is very intense study wise, so anything that you can do to reduce your workload prior to your start date will only benefit you. Once you begin the program you’re going to have a lot of other material (pertaining to your actual flying lessons) to read, memorize and study, so my advice would be to knockout the tests while the information is fresh in your mind.
As I stated earlier, if you cannot take the test prior to starting the most important thing to do is not let it stress you out. Set aside a certain amount of hours in the day to study for the written tests and complete practice exams. One positive that I can say in regards to taking the PAR after my start date is that a lot of the information made more sense once I began my actual flying, so either way you’ll be fine as long as you apply yourself.