Plains tiles should be largely avoided until very late in the game. My favorite thing to do with plains is to cottage in a city with a food surplus sufficiently high to support. Most of the time, though, the plains just remain unworked.
Post-biology, plains can be farmed for a +1 surplus in food. Pre-biology, farmed plains is whip fodder, not to mention wasted worker turns (what's worst is that farming is a worker-turn costly operation).