A little guidance...

[sooooo]

certain tiles - such as plains hill, river tundra, non-river plains etc, are considered simply worthless. It's not a question of what you do with it - you just don't do anything with it and leave it fallow, unworked.

Blake, you don't cottage plains tiles? Even if there are 2 food resources in the city? I'm guessing you'd prefer a specialist, but I'm interested in your reasoning.

[Blake]

Mmmpph. I'll cottage plains if there is food, but often there isn't, or i'd rather keep the food surplus for poprush. Usually I end up farming a majority of plains, with bored workers, as it gives the workforce somewhere to chill while waiting to die by the whip. And come biology, it kicks ass and some plains can be watermill/workshopped.

Really in the earlier game, all the drier and more marginal terrain is only useful to the extent that you can find easy food from resources to work it... because farming is a pretty poor source of food. I only farm to raise food surplus, to either:
1) Grow faster because there's no food resource (a city without 4f surplus is pretty weak).
2) Subsidize the workage of a very good but foodless tile - like gold mine. Altough even then it might be that 2 grassland cottages are better than 2 grassland farms and a goldmine... ie the goldmine is 3h 8c - split over 3 workers, that's 1h 2.7c each... a cottage will be 0h 5c each... and 3 grassland cottage will (eventually) be 15c...
goldmines are often overrated. They are good in the early game for a shot in the arm of commerce and happiness (and nearly always better than resourceless mines) but in terms of commerce the cottage is still king.

Grassland Gems otoh are very good because the tile feeds itself. That is massive. A grassland gem mine is very close to an end game cottage in yield. Probably the best commerce resource in the game.