Exactly. The only "right" way to choose active vs. passive is to decide what you want to emphasize.
When describing an experimental method, it literally does not matter if a Nobel laureate, fresh-faced undergrad, robot, or zombie added the reagent. The point is that it was added, and so the passive voice works fine.
However, perhaps it is important that the reader understands how carefully patients were evaluated, rather than the fact that they were evaluated at all. Then, you might choose the active voice and write "A panel of 17 experts carefully evaluated each patient" instead of "patients were evaluated by 17 experts."
Indeed. Choosing one or the other is making a choice about what should be emphasised. Any form of writing is subjective, and scientific writing is not different.
