Great question. I know each lab has their own opinions on this topic and there isn't necessarily a "right" answer but here is my thoughts:
We try to keep the gain as low as possible while making sure the needed LED excitation intensity isn't causing photobleaching across the time span we are aiming to record for. For the V4 Miniscope, likely LED values under 30 and recording lengths under 30 minutes will not generate noticeable bleaching. That said, the only real issue with always increasing the gain over increasing LED intensity is the gain will scale up both the signal and CMOS image sensor noise. This will result in slightly noisier recordings where things like salt-and-pepper noise and potentially some row noise might show up. But these are very easy to deal with and filter out offline so they aren't much of an issue.