Say that you have military spending at 50%, and social and research at 25%. You have a planet with no starport. Does that planet miss out on a potential 50% extra social/research production? Or is it smart enough to divy it out to social and research? If its going to waste that way, is there a way to manage economic spending on a per planet basis?
It's not really going to waste. Let's say you have Military and Social at 33% and Research at 34%. If you're not building anything, the 33% dedicated to funds are "returned" to you each turn, and Social and Research remain unaffected.
That is, if you're earning 100 bc/turn, 33 bc/turn would go to Social, and 34 bc/turn to Research. You would be spending 67 bc/turn.
Now, if you turn down the Military slider to zero, the other sliders re-adjust themselves proportionally. So now, you would have 49 bc/turn dedicated to Social, and 51 bc/turn to Research, and you're spending 100 bc/turn.