I've only been playing a month or so and I've come to the conclusion that any of the guys that I happen to catch with the right ability and solid stats I will wait to evolve. Anything else I'm just using to progress through the story I evolve when I feel I'm starting to hit a wall and have trouble with gyms.
I do suggest catching yourself a good gastly and holding off on evolving him until late 90s. He is very useful for the elite 4 in both hoenn and johto and being able to easily max his level will be very beneficial