The long or short of it?  The castle is thought to be a place of safety for the King, and our advice to new players is to castle early and often.

For mature players it is much more complicated, and it is important to know when to castle early, and when to postpone the option, or ignore it altogether.

Those who have played me, know I play the King's gambit whenever I get the chance.  One of my favorite lines is in the Muzio gambit when on move 3 Black prevents me from castling by playing Qh4+, 4. Kf1.  White's rook is entombed in the corner!  Hasn't Black secured a positional advantage?

Are there other openings you like to play, where you plan on not castling, or where it is often preferable not to do so?  

One of the most common mistakes I see in low and mid level players is castling after having entered the endgame, when the King is more powerfully placed in the center of the board.

In other openings, the castle becomes subject to seige, and the  King may enjoy greater safety elsewhere, even in the middle game.  Let's discuss some of those situations as well.

Well I've started the ball rolling.  Let's hear ideas from others.

WP  

My understanding is to try to avoid extremes. Castling too early gives away the kings address too soon and allows the opponent to position his/her pieces to target the king. Leaving it too late can leave the king in a great disadvantage in the centre with a greater vulnerability to attack. It all depends on how confident one may feel to castle later than sooner.And of course there are those instances where it is inappropriate to  castle at all.

 I expect most of us to see castling as something where we try to extract as much as possible from one move. That is, we move our king out of harms way into a snug and secure position. Our rook is not only brought into play but it may also attack an opponent's piece. The two rooks are also aligned to greatly increase their strengths. There may also be other elements at play. One needs to be flexible and adapt to many circumstances. In the end, like any other move, it reflects our personality.

Castling occurs once in a game and fixes the long-term residence of the King. This makes it an important strategic decision.

For offense, aiming the pieces at the opponent's King is a common strategy, so knowing the address of the King helps to develop the forces. For defense, keeping a piece or two near the King is also a common strategy, so knowing the address of the King helps here, too. All depends how confident you are to attack the other players king.

Castling occurs once in a game and fixes the long-term residence of the King. This makes it an important strategic decision.

For offense, aiming the pieces at the opponent's King is a common strategy, so knowing the address of the King helps to develop the forces. For defense, keeping a piece or two near the King is also a common strategy, so knowing the address of the King helps here, too. All depends how confident you are to attack the other players king. Also the position you are in.

Castling occurs once in a game and fixes the long-term residence of the King. This makes it an important strategic decision.

For offense, aiming the pieces at the opponent's King is a common strategy, so knowing the address of the King helps to develop the forces. For defense, keeping a piece or two near the King is also a common strategy, so knowing the address of the King helps here, too. All depends how confident you are to attack the other players king. Also the position you are in. HD

Some variations of the Kings Indian see Black castling very early, getting the bishop out of the way by fianchetto, and the knight to f6, and castling on move 4.

Often Black will castle in 4 or 5 moves in the Dutch defense.  Other openings seek to discourage or restrict the opponent's normal castling, for example the Benko gambit.

Probably as important as tucking the King away, is the development of the Rooks, connecting them, and preparing them for action, hopefully on center or near center files. or whereever files become open or half open.