Sequel Numbering

There's something I always wondered about regarding sequels - how much time for consideration do they put into deciding how to number sequels? Are there producers and writers spending hours arguing over whether they should use Roman or Arabic numerals? For the Saw series, they used Roman numerals by design, because then they could increment with severed body parts, but for many other cases it's bound to be more arbitrary. I wonder if they make that decision when they start a possible series, or when they make the second one.

That's the job I want in the film industry... Determiner of Sequel Numbering.