Yes, I know about white dresses! It becomes a nightmare as meshmixup says
But if the white dress has lots of material texture like stitching or small holes, etc. it helps a lot.
I dont think I would recommend green background since any color close to the subjects bleeds out and the dresses will come out with a slight green tint unless you have the background pretty far from them.
At least that is my experience, I don't know if anyone is using a colored background successfully?
I would guess a possibility is to have a mid grey background?
Also very important, try to go away from auto focus. It is not as important in a constant light situation as yours compared to the flashes but still you can get a few cameras that didn't focus correctly on each session and you will loose important info for the PS process.
You should get someone with nice easy to focus clothes (jeans for example) in the center of the shooting and manually (or automatically) focus all the cameras. When you are sure all of them are well focused (you should zoom in at least 200% to verify) then you turn them to manual focus and leave them that way.
The masking can be tuned per chunk or per photo if you want. Just use the tolerance slider on the import masks window before applying the mask and test a few different settings until you get the best possible one. After that you might have to do some manual tweaks on each photo anyway.
Leo