You will find many threads on this in the archive.
Basically, it depends on which country you are landing in/departing from. It seems that some require shades UP and some require shades DOWN. Overall, it appears to be UP with cabin lights dimmed so that your eyes are accustomed to the dark if you have to get out quick and also, the dimmed cabin lights will not overpower the navigation lights of the a/c, when viewed across the field.
There may be other reasons but this question usually collects half a dozen answers from around the globe.