Her Ult is basically made for the sole purpose of gathering the enemy team and have others on your team pop their ults to finish them off. She can put out some damage herself too, but it's not worth using (usually) if she's alone. So always make sure someone else has their charge ready to go, and are willing to coordinate with teammates (see my post above). As far as shielding, I try not to use it when I'm getting hit less frequently. I wanna make sure I get a decent charge out of it, but I also don't wanna commit so much that it gets busted and I'm suddenly dead. Full charge is useless if you're killed two seconds later.This is from Xbox perspective, so YMMV. Tracers always seem to shoot no matter what, so if one is harrassing me I'll pop it on myself or a nearby healer and get some nice charge out of it, with little risk to myself. Always have your shield ready on a big push. Don't be afraid to bubble DPS guys on your team, too, particularly one that is about to or is already using an ult. Soldier and Genji with a bubble is big trouble for the enemy team, and they instantly become the focus of their DPS. Therefore, you get more charge because they're going ham on the person using their ult.
I know that's a bit disorganized but that's how I see things when I'm playing. More experienced players may differ.