Well, I think jorge_roberto may be referring to when you play on a cramped map (lots of civs with not much land available), or towards the end of the expansion phase where you are left with few choices because there is no more room to expand.
It all depends on how close my neighbors are. If I know I have the time, I will go for the good city site first (bonus resources, river, etc.), because I want those cities set up right away. However, if it looks like the AI may grab a resource that I really need then I'll grab the resource. If I already have horses, I'm not too worried about the AI also claiming horses. Iron may be a different story. I don't really use swordsmen, but I'll eventually need iron for my knights, and it's nice to deprieve the AI of swordsmen and pikeman.