The key is to do what sounds best for the particular project you're working on. Vocal layering is done for many reasons, and it's never done because so & so is doing it. Most of the time it's used to hide flaws.
Sometimes it can be used to get a stereo effect (wide vocals, guitars etc.), other times it's to get a choir effect by recording the same vocal part with a tenor voice, soprano and then final one maybe use alto. One vocalist can sound like a choir, but this will require maybe 2/3 soprano vocals, 2/3 bass etc.
And taste does play a role as well. So my advise would be focus on what will work best for a particular project. If something needs layering, you'll know, trust me on that, you'll know. Think of layering as a solution to fix certain problems instead of a compulsory technique.
Always have the end goal in mind coz layering can make mixing a song very hard.
Hope that helps