power up strategy?

Defenders first...is the rule I follow. These are where you will get most benefit from having a higher lvl/CP

Prestigers never get dust. It is too valuable to waste - for prestigers simply catch one that will evolve to the CP range you need it to be. IV is less important, and so getting the right CP range, with the right moves is far more important. A decent prestige team can be built without spending dust, and only using candy to evolve...not power up.

Saving your dust...well that part is a personal decision. Some say, live for now and play the game we have - others will tell you not to do anything.
Personally, I am going for a mix of both - that is, if something needs dust/candy then give it to it, but I am keeping a little in reserve as insurance (around 200k minimum stardust) so I can react should things change.

These are just my way of doing it, I am sure others have a different preference that suits their play style.

I max out my "super mons" meaning high IV, meta relevant, and top moveset regardless of whether they are attackers or defenders. Why? Because it's fun.

If you have a good Snorlax or Blissey to take past lvl 30 I would. I would not take any Rhydons or Tyranitars past just for placement because who knows what will happen with the Rework because if cp aint on their side they got nothing. But Dragonite and Gyarados are definently the best of the big 4 Gyarados being slightly ahead in my opinion.

Army of Pidgey!

I have maxed out the best general attackers, dragonite, exeggutor, snorlax, blissey, vaporeon.

With these maxed, I can take down gyms really potion efficient. For me, is much more fun to take gyms from time to time in comparison to have some maxed defenders sitting somewhere in the outskirts and earning money quietly.