So should you always hold your 100% rail until there's a kill opportunity?

I've heard people always say not to use rail early but I've assumed that meant wait until high charge and a good target. However there's plenty of times where I charge rail to 100 and there's no opportunity to kill a squishy prefight.

It feels really weird to hold on to a full charge rail 20 seconds prefight when I could just shoot it getting someone low and getting some ult charge. I'm diamond currently with a 70% winrate so I'm not doing terribly I just think I misunderstood something really basic about her lol.