You should never *need* to use Flash on single targets. Just emnity combos are fine. Groups (trash mobs and adds), yes, you'll use Flash a lot. Once you have an aggro lead use your combo, but rotate it around the mobs based on who your DPS are targeting.

As for your CDs, it depends on the fight. For dungeon bosses, there really aren't many tank-killers, so just rotating CDs when you start getting low or when adds spawn is fine. Later, when you start seeing tank-killers, you'll need to plan on what CD you'll use and when. For groups, its always a good idea to use a CD after pulling a group to help out your healer.

And Shield Lob is always a good opener, groups or bosses.