Evolve Wailmer?

I don't think Wailmer has any chance of being much more than a Dex filler. The only possible (and unlikely) issue is they could do an 'evolve Wailmer' task as part of a Special Research event (they've only done a 400 candy evolution once though and that required much more common magikarp).

I would wait at least until Jirachi special research comes out. It could very well involve evolving both Wailord and Altaria since they are gen 3 pokemon.

If you're interested in PvP at all, make sure you pick one that will be no more than 1500 CP. Wailord has som use there because of a huge HP stat (4th highest in the game) and a diverse set of moves.

The bigger worry that was already pointed out is if part of an eventual Jirachi special research requires you to evolve a Wailmer since the Mew research required you to evolve a Magikarp. Swablu could also fall into this camp, so I'd hold off on evolving one until either that research comes or until you have 800 candy so you have a constant baseline of 400.

Personally, I would hang onto it, until you get enough candy for a second one (or close too it). Niantic did it with Magikarp, and I strongly suspect they'll do it again sometime. Wailord isn't an especially useful pokemon anyway, so it's not like you're missing out on much by waiting.

I'm holding onto Swablu candies for the same reason (though in both cases I already have the evolved forms, I'm just gathering more candies so I'm ready for future quests).