Fun fact: You can evolve Porygon at level 1 and it's ok as it doesn't really lose any good moves doing so, so I have a level 1 Porygon-Z in my living dex.
I think Pokémon that have multi-generational evolutions can get away with not being evolved over Pokémon whose evolutions all appear in the same generation. Likewise it's safer to evolve them early.
For me the exception is pretty much all second and sixth generation evolutions or previous generation Pokémon including Steelix, Blissey, Espeon, Porygon2, Umbreon, Scizor and Sylveon.