Unless they have forests on them, I generally avoid working plains tiles until one of three things happen:
1) I have State Property, which makes Watermills turn the tile self-sufficient and Workshops not remove all food. This is good for a production city, and the Watermills even give some production to a commerce city in urgent need of it (say, if you've had to shift to Emancipation).
2) I've already worked all the grassland and flood plains tiles, and all the food resources, and I want to spam cottages for a commerce focused city. I only do this when I have plenty of extra food elsewhere in the fat cross, however, otherwise I'd wait for Biology and then alternate farms and cottages.
3) I have Biology making a 3/1/0 or 3/1/1 tile practical for a food starved city. I'd still wait until the better food tiles are all worked, however.