Actually this has been talked about by a lot of Pokemon Go users. And while it's true when evolving, the game applies a set multiplier to your existing CP, but the CP bar of the pokemon stays the same through evolution, which means it doesn't matter as much whether you boost it up before or after evolution! As this post stated: "When you evolve a Pokemon, its CP circle stays as full as it did before. As a result, it really doesn't matter in which order you boost a Pokemon."
What you wanna keep an eye on is the stardust cost. The higher your pokemon's CP is, the more stardust it will cost. Hence why it's better to boost your pokemon up to max CP first before evolving in order to save stardust if you have enough candies to do so! But this is a risky move because you may may as well come across an even more higher CP pokemon in the nature.
Someone actually disprove this theory with a video of them evolving an eevee with the name RAINER, but it turned out into a Jolteon. I also heard someone said it only works once per account. I think the more accurate one is the moveset. If Eevee has "Dig", it has higher chance to become a Flareon. "Swift" for Vaporeon and "Body Slam" for Jolteon.
On another note, I don't really dedicate much time to the game yet LMFAO just a filthy casual player haha.