There is no such concept as “groceries getting bagged for you” in Germany. I have a backpack with me where I put my groceries.
Regarding your question, yes have a strategy.
The basic order on the belt is heavy to light items, so that the heavy things such cans or glas bottles go to the bottom, light stuff like yoghurt and eggs at the end of the belt so they come on top of the other groceries.
Of course this is not fixed, as light but bulky items may get a prioritized place on the belt. The worst thing that can happen is that you have to repack your backback.
However this is not all. As our cashiers are usually professionals, you will need to stategically slow them down, you want to avoid the shameful and pressuring looks of your successors. I do that by putting items inbetween the other stuff on the belt that have to be counted or weighed, such as pastry and vegetables. This gives you time to pack your stuff or rearrange in case you made mistake a step earlier.
It’s true Germans are not known for being very patient and easily get annoyed when standing in line, but I may have exaggerated a bit ;)