I've never noticed any difference foraging from one place in a zone and foraging another place in the same zone.
As far as I know, you can be anywhere in the entire zone and get the same resuls foraging (same table of items with same random chance for any item). Also, as far as I know, zones have tables of items that can be foraged. I do not think that any particular item on that table of items is more or less likely than any other item on the table.
Some zones are better for some items than other zones too. This is mainly due to the number of items on the table for any particular zone. I always prefer foraging direwolf tufts in Kael to foraging them in say dwarf town or EW. There are more things that can be foraged in the other zones, so it is less often that I would get a tuft there. Kael only has a few things on the table, so tufts come up more often comparatively.