Is there a way to determine if specific micro is worth it?

I agree with dphil except for the order precedence of micro and macro.

He says that you should always micro and then try to remember to macro I believe that the opposite is better for both learning and being a better player.

I have proved the importance of macro over micro multiple times to my lower league students (bronze - gold) where I would defeat them without using ANY micro at all. I would focus solely on macro and just attack move my army in their general direction. Sure they might withstand the first attack or two because their micro is superior to my own but their macro is quite poor because of this while mine is still as strong as ever. So my army eventually just ends up overwhelming them.

Being able to macro well is hard in itself, once you get that down then you should add more and more micro while not weakening your macro.

I think that you are correct that even at higher levels of play there are limits and that focus on one or the other is more important; macro is usually (there are always exceptions, especially in the case of cheeses) far more important in the early game whereas micro will usually be more important in the late game.

As to your question, same as what dphil said, you can't determine exactly. It depends on the game and lots of factors, uploading a replay and getting someone experienced to look at it might be your best bet. As a general rule, I'd say micro over macro is rarely worth it if it doesn't win you or prevent you from losing the game right then.

Yes and no. It is possible to determine whether or not specific actions were worth the time and attention, just like it is possible to determine if certain actions were even worth doing. Both are handled differently however. People will argue that one is more important than the other, this all comes down to your play style.

Actions worth the time: This is something you have to be conscious of because many actions can also be made with hotkey usage, without ever needing to adjust your screen position. In your reaper example, you may spend 10 seconds microing your reaper but you should only spend 1 second clicking your command center hotkey, pressing 's', and then clicking the hotkey to re-select your reaper. All of which can be done without you having to move your screen away from your reaper.

With that same scenario, if you were supply capped, then it would be a higher priority to back your reaper off to safety, manage your supply depo production, and then return to the reaper management. From a replay you will say "yes I needed to do this instead", but that does not always mean you should neglect your micro, a lot of times they can synergize.

Actions worth the Investment: This is something you identify much easier in replays. Say I have an army of 3 mauraders and 7 marines, and my opponent has 16 zerglings. My opponent comes rushing into my base to engage my army and I react by attempting to studder-step my way back. In the replay I see our armies before they collide, and I see the outcome after the fight is over. From there I can say "Well studder-stepping delayed the fight, however my execution caused me to lose out on damage". If I repeat the scenario with the same numbers and do not studder-step then I am capable of judging whether or not the micro had hurt or helped me.

If your mauraders are in front tanking sometimes it is better to hold your ground, often you will see terran players actually lose more because they attempt to studder-step. Same is true with blink stalkers and speed lings.