If it's more than 50% green, don't touch it.
If it's less than 50% green, give it a gentle tug. If it drops off, the plant was done with it anyway. If not, you can leave it, because some nutrients, like Nitrogen, are 'mobile' and the plant can actually suck them out of dying leaves and recycle them elsewhere.

Edit: I just noticed your siggy... very cool. Food for thought.