Excellent question. For a pico reef, that answer is almost the difference between life and death. In larger systems, it is more forgiving but the answer is still in the morning, strictly, if you want the smoothest possible chemical transition.
High-pH (evening or afternoon dosings, after day-long photosynthesis has removed available carbon dioxide from the water thereby driving up the pH) closed bodies of water aren't very accepting of calcium and alk dosers (also conveying pH upping characters {primarily the alk side}) when compared to their most receptive period, the mornings before lights on. During the night phase and the absence of photosynthesis, CO2/carbonic acid levels are highest (pH lowest) and that's the time to add the C-Balance. Obviously the early night phase is still a transition time between high and low pH periods, so the morning is the best.
I have killed year-old acropora growths in my picos by adding C-Balance in the after noon, before someone helped me with this same concept. I also know of people who add it to their large systems continually, or at night, and things don't die because the impact to systemic pH isn't all that notable, but it's still there. It's better chemical timing, and it's easier on your system in habitants to not add dosers in the evening or in the afternoon. I suspect that constant-feed systems, in small increments all day long, are less impactful that the full addition after lights have been on for a while...good luck getting through all these parenthesis Sir ( )
B