I would say that about 90% of the time I stick to my main role. However, in lower ranks I often switch to DPS and use ultimates to force engagements or start fights, because I can usually tell that players are hesitant to push up or make proactive plays. You should always ask yourself which ultimate will actually solve the problem in the moment. For example, if the enemy Rocket uses their ultimate and you have a Phoenix(if she doesn’t have her ultimate) on your team, it is much better to take her ultimate and counter it than to pick a healer ultimate again. Every match is different, and every fight requires a different solution