It's situational, but sometimes it's also a matter of psychology. Guarded healer feels more safe and thinks you're competent, thus he'll go more all out to heal you AND the DPS, rather than just you and otherwise avoid generating threat.
Only time when healer gets aggro is when nobody is attacking the mobs or tank doesn't have enough threat over the mobs. For example beast boss in Athiss: adds that spawn will go for healer if tank isn't close enough to them or dps isn't attacking them (like they should do!). Threat healers generate is so pathetic that even ranged dps will steal the aggro from healer just by sneezing in general direction of mobs.
"Always Guard the healer" only works in early FPs (mainly Hammer station, maybe Athiss). Guard is situational ability that should be used when it helps (especially with healing). Don't ever Guard the tank though. I have to learn to use that more often.