I'd advise against investing in anything unless it matches current raid bosses (as in the current raid bosses when gen IV's released). Powering up something for something as unpredictable as lower level raids is like finding a needle in a haystack, theres too many possibilities (personally, I think they'd go the route of making most of them pokmon that got gen IV evolutions as many are/were already raid bosses) Also, most of that stardust and candy could go to waste as gen IV is coming soon, and theres no telling what will be good and what wont be (the guide on gen IV does assume best conditions, which isnt likely to happen), so depleting stardust before that comes out could put you at a disadvantage if something that outclasses what you've powered up is released, or what you've powered up turns out to be ineffective against raid bosses (though this mostly applies for tier 2 and 3)
TL;DR, save powering up until gen IV's released, then make you power up decisions from there