The hardest times to keep aggro on all the mobs are when there's lots of them, and that tends to come up with groups of mobs that are (at least individually) relatively easier than normal for the dungeon level. While a healer may have trouble if getting hit by a boss, they can survive a blow or two from the typical trash mobs that come in groups like that. (After all, they have to be tough enough to survive soloing landscape content.) The bigger issue is that it interrupts their heals. So if you're in urgent need of a lot of healing, then you need to keep all mobs off the healer. If not, then you mainly just need to keep powerful mobs off the healer.

Certainly if a mob breaks away and heads for the healer you want to shield lob it back to you, but some people fret more than is really warranted over the mere fact that its aggro started to shift away even momentarily.

p.s. Yes, this is based on low-level dungeons, but I've already seen the attitude crop up that even a minor mob hitting the healer just once or twice is somehow an issue, and no it really isn't. A boss hitting the healer even once would be an issue. Lots of mobs ganging up on the healer would be an issue. But a minor mob getting a blow or two in before the tank pulls it away - so what.