Always capture toward the center? [FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Nc3 Bb4 4. a3 Bxc3 5. bxc3 (5. dxc3)


I usually see bxc3 in this position and the main reason given is to capture toward the center, but doesn't bxc3 weaken the queen side and take the option of castling queen side away from white.  What is wrong with dxc3?  Doesn't this open up the file for the Queen and the Bishop's diagonal?
Either is fine. bxc3 gives you a strong chance to push in the center successfully. It leaves a dangling pawn on a2, but by playing bxc3 you should already have resigned this pawn's fate to some sacrifice for a tempo later in the game. Note you can not play 6. d4 right away since black's response will most likely be 5. ... d5 or 5. ... Nf6 putting pressure on the e4 pawn which is awkward to defend for you at this point. You should most likely respond 6. d3 which dashes any hopes you had for a pawn charge to d4. Also, black can exert pressure on the doubled pawn at c3 which is also a bit awkward for you to defend. 

I personally would play dxc3. It is less committing. You no longer have to hope to pry open the middle with your pawn majority, and you have no dangling pawn you have to sacrifice eventually. You've opened up strong lines for all three of your white bishop, your dark bishop and your queen. Note this may seem not that important, but keep in mind your opponent just traded one of his bishops for your knight. If you can open up this game, you will have a strong edge on your opponent. Any attack on your e4 pawn can now be refuted, such as 5. ... Nf6 comes in with a strong pin 6. bg5. Also, it no longer looks silly for your white bishop to travel to d3 to defend e4.