While it is definitely advisable to hold off on powering up your Pokemon early on, when to decide to use stardust is highly situational. There are several things to consider:
- Does your Pokemon have high or perfect internal values (IVs) and desirable attacks?
- Are you a high enough level where the Pokemon you are catching or hatching now are not keeping up?
- Can you compete for gyms with the Pokemon you have?
If I were to ever acquire a desirable Pokemon with a desirable move set and perfect IVs, I'll eventually power it up as resources allow no matter what CP it starts off with.
Speaking for my own experience, I'm level 25 and I've started powering up the Pokemon I use to take and defend gyms... which are mainly Vaporeons (I have a small army of them). Most of them have high IVs and desirable moves (either Hydropump or Aqua Tail). I've stated to fall behind in level a little bit compared to opposing trainers, probably because I've yet to spend a single cent on the game, so all of my lucky eggs, lures, and incenses are from leveling up. So, I've been maxing out some nearly perfect Vaporeons to keep up with the level 28-30 trainers I sometimes see controlling gyms.
But keep in mind, this is simply for the situation I find myself in. If you're still finding Pokemon with high CP and you're keeping pace with the other players around you, there's probably no real reason to spend stardust quite yet. And if you're still at a low level, then you probably want to prioritize gaining trainer experience over increasing your Pokemon's strength so that you can continue to accumulate stardust for when you catch up and want to be competitive at your local gyms.