The main reason you should do at least 99% of your aiming while standing is that this way when you step into the shot in a repeatable manner, your entire body is synced up with the correct shot line the same way each time. Head, arm, body, legs, everything the same. Whereas if you start changing something in your aim when you are down, different components of your body are no longer in sync and you will have unwanted variance, leading to more inconsistency.
Moving the bridge hand fingers slightly is fine while down (to fine-tune the tip position on CB), since it doesn't affect the rest of your body whatsoever, but pretty much anything else (head, shooting arm, body, legs) moving sideways will cause a chain reaction of other unwanted movements. For example, you might sense that you need to cut a bit more thin. You move your shooting hand a bit to the side, causing the cue to be on the correct line, but now your body isn't in its normal position relative to the shot anymore, since you moved the arm sideways but kept everything else the same.
Even if you do the same sideways motion with your entire body to try to keep everything the same relative to eachother, you will not be as consistent, it's just not realistic to do that. Think long pots in snooker, you never see the pros move their entire body or shooting arm sideways while down to change their aim.
I said 99%, not 100%, because subconscious midjoadjustments are a thing, and also because the details of this convo start being more related to your specific way of aiming and aligning. But yeah, in general aim the cut while up, verify it while down, shoot if it's ok, get up if not.
