No, it will not be perfect in every setting. I mean this politely: It's a bicycle, not the space shuttle. The reason you find so many differing opinions as to the repair and upkeep of bikes is that they are forgiving of bad maintenance and poorly executed repairs. There is a lot of "play" relative to each part.
For example, sprinting causes the bottom bracket to be pushed laterally in both directions, constantly shifting the angle that the chainring presents to the cassette. If they were precisely mated this would cause ghost shifts at a critical moment. But it doesn't happen because they purposefully account for such variances in angles and fit.
In regards to the rear derailleur, if it hangs roughly 90 degrees under one cog, that relationship should be found amongst all cogs. The exception will occur under shifting.
The derailleur is nothing more than a parallelogram that keeps a constant distance from the cogs. If this does not happen it means that:
1)The rear derailleur hanger is most likely bent.
2)The derailleur itself is bent, but not as likely.