It really isn't about spaces. It's about the expected metagame.
When it was all Hogs, castles were buried (sometimes even unlurable) and contained a lot of Archers and a Wizard or two.
Since the nerf, the metagame has changed to mostly Dragons and GoWiWi/GoWiPe. For Dragons I would suggest not centralizing your castles and put in a Dragon. For GoWiWi/GoWiPe I would say centralize your castle and throw in some. Doesn't really matter much.
On lower levels a Castle is lurable and it isn't all Dragons coming your way, so I doesn't really matter much either. Any troop can be lured and killed with Barbs and Archers.
Thanks Dino and Jos, Seems to be the best CC troops for LVL 2/3 CC are archers/wizard combo...
Is PEKKA effective on LVL 4 CC? unless attacker do an air raid...