It sounds like the tanks you've run with have no idea how to hold aggro. Generating aggro on mobs isn't just flash once and done. There's a lot more to it. As a CNJ, I had no problems healing without pulling mobs off tanks as long as I was conservative with my heals. If you that you're pulling mobs off the tank, ask him to use flash a bit more liberally and also to rotate through mobs instead of focussing on only the main one. Tanks need to be constantly rotating through mobs to ensure that they're top aggro of all of them.