I try to save most of my forests until after Math, when they're worth more hammers. Most new cities do build a monument and granary (and workboats if applicaple) out of lumber, however, and I might chop more depending on the particular game. I do generally save non-hill non-freshwater tundra forests, since a lumbermill is the only improvement for that tile. And maybe one game in five I'll have a floodplains city that actually needs to worry about health.