Bg5 for White Before he's castled - Don't do it?

i read recently one of the old champion's axioms was "Don't move Bg5, pinning blacks Knight BEFORE he has castled" as some solid general advice.  Is this true?  

As advice in specific openings, probably. As general advice? -- much too general to take as a "rule of thumb" in my opinion.

Well, I mean in all variations of the Queen's gambit declined, white plays bg5 before black has castled. also in the classical variation of the french defence this happens.

It may be a good advice in a certain opening. But you can't take it as a rule, there are many openings where white plays Bg5.

There is no reason for such a rule of thumb.  There may be a couple of positions where it is a trap, and obviously it can be chased by the pawns (this can be good for the side with the bishop, as the appeal of overextension is hard to resist for some players) but so long as you do not totally blunder the move (example, get it trapped by  your own pawns and killed by his pawns) it can be a good move in many games.

it's probably referring to the italian game, where black can play h6 and g5 with tempo, castle long, and have a head start in pawnstorming the white king.

Yes. There Bg5 is very dangerous for white-