Wailmer, would you only evolve high IV Shinies or does it not matter because they're shiny? I know you only keep the best usually 
SneakyGremlins Shinies are the exception to my 90%+ IV rule because they’re so rare 
If I have more than one, I will pick the best IV out of them. I’ve got two shiny Roselia so I will be evolving the best IV of the two.
I also don’t always evolve a shiny. If I only have the one, I will keep it in whatever stage I prefer the look of. For example, when I just had one shiny Wailmer, I kept it as a shiny Wailmer rather than evolving to shiny Wailord as I preferred the look of shiny Wailmer
Only when I got a second did I evolve one.