The question with Suicune is not should I power this one, but should I power it at all? Vaporeon is generally considered a stronger water attacker. Further, there is chatter that Gyarados might get a water quick move when Gen3 is released, which would make Gary comparable to, if not better than, Vaporeon. And eventually, Kyogre will be out there (legendary water), and he should stomp Suicune's butt. I'd save the rare candy and stardust, unless it's burning a hole in your bag. Even then, there's other places you can put it to better use.
Of course, this is a game, and it's all about having fun. If it would be fun for you to power up Suicune, go for it. From a strategic point of view, it's not optimal. Your call.
