IMHO, anytime between 4:00p and 5:45p.
I prefer eating while the park is closed and finishing my meal around 6:45p so I'm NOT herded to Tomorrowland so I always make my PS between 5:00 - 5:30.
If you make an earlier PS, around 4:00 - 4:30 you can do rides afterwards in Tomorrowland before the party even starts!