So, in sum, I really believe that mounting an early offensive is a huge inefficiency. Particularly if your greatest threat is some distance away from you.
The process that seems to work without fail for me is to expand rapidly, build infrastructure, and THEN conquer the closest civ that is about the same size as yours, when you get cannons - earlier only when there is a civ out there that is just running away with the title or you have a nice tech advantage - and then aim for the biggest threat (unless you just conquered him of course).
edit: Of course, map size should greatly affect strategy. I favor huge maps and bigger. I imagine that the smaller the map, the more appropriate an early offensive is, due to the limited amount of space.