I place much less importance on IV since the latest changes to gym prestige. The 18% difference that would make your Snorlax 100% will never make the difference between winning and losing a gym fight, realistically. Therefore I'd just go ahead and power it up. The only exception maybe being if you get a lot of Snorlax in your area and you need to prioritise the candy on maxing out the best one.
I have a 76% Dratini I'm probably going to have to resort to powering up to my first Dragonite. Dratini are so rare around me that I doubt I'll get much better, and even if I do, a Lapras/Dewgong/Cloyster will still go through it like a knife through butter.
And congrats on that 100% Spearow