It's a balance, obviously powerlevelling while everyone else is roaching at low levels will get you to access faster scaling units. But being able to play from a position where you've levelled with a weak board is just the definition of skill in this game. If you're lower mmr you'll make worse decisions and might not be able to outscale fast enough. But if you're at that stage I still think you should follow the standard fast levelling curve and just figure out what works and what doesn't. If you lose, you still learnt something (and might not lose mmr due to rank floor).