Does it usualy matter?

Whether you castle on the same side as your opponent or not? Generally speaking, I mean.

Yes, it often dictates the middlegame

If both players castle on opposite sides, it often becomes sharper due to pawn storms on both wings, espcially if the center is closed. If the center can be blasted open, then these attacks won't really work. 

Castling on the opposite side is like declairing war.

 Good god.

Anymore?

Isn't just sitting down at the board like declaring war?

No, generally speaking, LavaRook said it all.

Yep, the "typical" opposite-side castling situation is mutual pawn storms. In any given middlegame, it would be great if you could just throw your pawns forward and try to break up the enemy king position. When you're castled on the same side, though, this often isn't a very good idea since it's your own king's pawn cover you're sending away! But when the kings are on opposite sides you can march forward the pawns on the wing opposite your king with much less risk. Since the pawn storm is such a potent way to attack, you usually have to pay close attention to the initiative in these kinds of positions and try to figure out whose attack will come through first. In fact, you want to start taking that into account before you even castle -- otherwise you can just castle into a losing attack that you have no hope of parrying.

Of course, that's just a general rule. In some positions a pawn storm still isn't the right plan, for example if the centre is wide open or if a counterattack there would be particularly nasty.

So is it generally a good idea?

That depends entirely on the position, based on principles already touched on by some of the previous posters.