Don't pin the king's knight before opponent has castled?

I've heard this opening principle:

"Do not pin the adverse King Knight (ie. by Bg5) before your opponent has castled"

Why is this? Have any example game?

I'm not sure it's a problem if you do it without him castling, it just wont be as much of a threat. If your opponent has castled and you capture the knight without it being protected by another piece you will double the pawns by his king, which will expose it. If they haven't castled, they can accept doubled pawns on the kingside and castle queenside.

Or if they haven't castled they can simply chase the bishop back with the kingside pawns, and you don't have fishing pole style traps available.

I haven't heard this one, but the logic behind it is probably that you want to make it hard to remove the pin. After your opponent has castled h3 and g4 are sometimes necessary to remove the bishop and they are extremely weakening.

If you pin right away your opponent does not have to castle to that side and can use the g4 h3 to attack your king if you have castled kingside.

It really all depends on the opening though, sometimes it is better to trade a bishop for a knight if that bishop will be bad otherwise.

If I recall correctly, this advice comes from Steinitz via Lasker and probably should be taken with a grain of salt when it comes to many more modern openings as well.