who to guard should be decided by the situation...
There are precious few situations in this game where guarding the healer is the right solution for threat-related reasons (TfB comes to mind).
Other than that, the only "situation" that decides giving the healer guard is when they demand it and you don't feel like arguing.
If you, as a healer, are getting attacked, it's because nobody else has attacked that mob
. Guarding you doesn't magically make someone else attack the mob -- so guess what: You'll still be at the top of the threat table
That said, HMFPs can be done without using guard AT ALL. Just by being a good tank and judicious use of taunts, you can keep aggro. So frankly, there's no real harm
in guarding the healer, even if all you get is the 5% damage reduction.
But if you think that guarding the healer will solve a threat problem, you're mistaken (with a couple of exceptions in operation fights with very specific mechanics).