timing of topping?

[habitat_gardener]

I recall reading that some people top their plants toward the end of the season so that the greenies will ripen before the cold hits. I was wondering about the timing. How many weeks before the end of the season do you do this? Or do you just pinch out new blossoms after a certain date, and leave the foliage? Has anyone tried topping (or pinching) some plants and not others, and is there a difference?

I usually try to keep my plants going until frost hits at the end of November, but it's not really worth it for the larger varieties. I don't get much production after mid-October except for some cherry tomatoes and smaller varieties. Mid-Sept seems early to top them -- some of the later ones don't even start producing until then.