Flügel flach oder unter zehn Grad

I'd say it is mostly a matter of personal preference. At least for macrolepidoptera, which are almost always determined by looking at the wings. A slight angle may look a little nicer inside a cabinett. But if you plan on photographing your specimens, I'd recommend a flat view to avoid distorted wings. In any case I think 10° is a bit too much. Try 3-5° each wing.