There is a point in every photographers path where you think about the perfect level. There are discussions going on if the horizon should be levelled all the time. I have my own opinion on this topic.
There might be some reasons where the horizon should be levelled. These are optical reasons as I would call them:
- Panoramas or similar pictures
- Groups of people
- Picture of Water
- Things which are “always” horizontal or vertical
- …the reasons I certainly forgot
But you should keep one thing in mind: Your eye is very good in detecting angles. You will have problems to tell the difference between various lengths, but you will have the abilities to tell the difference between two angles which do not differ much.
And the other thing is: A photographers eye will be even more merciless.