I usually chop to the health limit in my capital as soon as I get BW, which is always one of the first techs I research. You need the hammers to get your Settlers and Workers out fast at the higher difficulty levels. Then I am usually looking to build Oracle, Pyramids, etc. and the chops help there, too. Once you get to Math and can build Aquaducts, that allows a little more chopping -- maybe even to help build Hanging Garden. If you have a chopped tile undeveloped next to a forested tile, it can eventually regrow the forest there. If I am close to getting Construction, I hold off chopping, because you'll get more hammers per chop post-Construction. When you start a new city, you will rarely go wrong chopping a couple green tiles, especially on rivers, to get it growing fast. My bottomline: cut a lot of trees, but timing is important, and clear-cutting is not a good idea for most cities.