Power Up

Always power up after you have the highest evolution due to the fact that moveset's are random.

If you don't care for moveset then it doesn't matter.

Whether you do it before or after will always give the same result.
However, waiting until after you evolve ensures you don't waste dust/candy on an unwanted moveset

Powering up before is like forcing yourself to learn guitar by buying an expensive one. I invest heavily before powering up, then you have no choice but to use the Pokémon regardless of moveset.