Some Banished knowledge bases (I think the game for one) seem to imply that herbalist/gatherer likes old untouched forests to do their work, but a lot of people recommend a forester to keep a dense forest cover next to your gatherer/etc (The wiki says both of these things, I believe).
So which is it? Is a forester a bad thing to put next to a gatherer, or the absolute best thing? Maybe a Forester does more good than ill, but the optimum would be a completely dense untouched forest?
"Old Growth" versus "New Growth" is not an actual mechanic. The only thing that matters for gatherers and herbalists are tree density (since this determines yields / spawnings of the smaller plants).
While I'm not certain on the absolute value of not having a cutting forester overlap a herbalist / gatherer, you definitely want to set up gathering spots in areas that have been planted / tended to by the forester, because forester's create a much denser copse than can be found on the initial map.
I'd wager your last paragraph has the right of it: a planting (but not cutting) forester + gatherer maximizes the gathering, while an active forester + gatherer maximizes total resources / map area.
A forester is not a bad thing at all, in fact you can set your forester to only Plant trees and not cut any down in order to provide the largest amount of "Mature" trees in the area, which will help you maximize the output from your Herbalist/Gatherers.
Personally I let my foresters run on Plant only until I have a nice dense forest for my Cluster to work from, after things stabalize I usually re-activate the Cut option to supplement my Lumber Supplies.
