The layered image and the flattened version are the same - there's no way to avoid that.
What can happen is that you see a noisy image at less than 100% zoom, and the preview and final can differ a bit (because the preview is downsampled). Or in 16 bit/channel you may get a slightly coarser preview at less than 100% zoom.
In other words, as long as I stay in the layered photoshop environment, all's good, but making any modification required by the outside world will result in degradation of what I see only within photoshop.
>just to make sure: With »flatten« You do mean reduce to the background layer and not just reducing the file-complexity by merging groups or such?
("reduce to the background layer") is not used in the English version of Photoshop. Not even
The menu item labeled "Auf die Hintergrundebene Reduzieren" (oder so was ähnliches) in German is labeled with a single word in English:
I know how hard it is to refer to specifics in a version in a different language when you don't have it available. It's not a question of just translating the word. "Layers" in the German version are not
layers at all but
planes (»Ebenen«, nicht »Schichten«). Took me a few seconds to figure that one out when I started out to learn Photoshop. :)
The concept of "Reducing" is not used in the English version at all. "Sichtbare Ebenen auf eine Ebene reduzieren" is simply