Although this happened on the other team, it applies to everyone. We were 8-3 up going into the last round. And all of a sudden I hear "I have got your trail", "You want to play let's play", "you are powerless", "get out of my way" and then finally "welcome to my world". They went on to win and make it 8-4 but I have a feeling if they had used some of these ults earlier, they could have won more rounds. Even winning one more round would have led the score to be 7-5 which is pretty good.
Use your ults. Best case scenario is you use your ult at the right time. If you don't know if it is the right time to ult, then just use your ult on buy rounds as soon as you can unless you are jett/chamber/neon. Using is as soon as possible is better than not using it.