Typically before. I/J decks are randomly given out. The second bullet point in the host guide implies that you find out what deck you received (“examine their decks”) when you already received the packs you picked.
From the host guide:
- To begin, each player is paired up with an opponent. Each player should have a starter deck and the selected packs.
- Players examine their decks and open their packs. Each player will have between seventy-four and eighty-five cards.
