To be honest, not having GMed before, I think this debate comes down to "is it using OOC knowledge to use knowledge of a common monster's weakness?"
In my view, as a person relatively new to PFS, no. There's perfectly valid reason for a character to have IC knowledge of a common creature. As others have said here, Pathfinders in the story, require training. At some point, the weaknesses and resistances of every day creatures would probably come up.
It's a little much to say that a person who's been at... Pathfinder University or whatever the term is, doesn't know how to deal with a skeleton. It's also a waste of everyone's time rolling knowledge checks for information the character should rightly have.
If it's a boss or rare monster that the character himself would have no valid reason to know about, I agree knowledge check required. But to me, it doesn't sound fun seeing a member of a party play stupid trying to deal with a monster he already knows how to handle while everyone around him gets mutilated.
And the party not having fun is far worse than the occasional, debatable, lapse into metagaming.