For the truly anal retentive (namely, me

), I transform all areas to grassland for maximum food and trade.
There are a few notable exceptions, though. I leave wheat, wine, and gold alone.
Wheat transforms to grassland, which provides less food. Mining (re-foresting) yields silk, which is good, too, but in late game "pet city" environments, the food to support extra citizens is better.
Wine transforms to wheat (I believe), but I love the combination of production and trade.
Gold transforms to coal, which provides terrific production, but the trade arrows from gold are just too hard to pass up.
When I get to the end of one of my games, the whole world is railroaded, farmlanded grassland!
The removal of engineers and terrain transformation is one of my big pet peeves about Civ3, :rant: but there are many other problems with that game as well.