healing aggro isn't so clear cut, it's also not been figured out completely.
one fact we do know is that you only get aggro from actual heals, not over heals. with the amount of damage enemies doing it caps the amount of aggro healers can actually generate quite a lot. and that damage will be split between other people too, for example a captain will heal off actual numbers a lot with smaller ticks while your minstrel's bigger heals might have large percentages of the heals over-healing and not turning into additional aggro.
in-combat moral regain makes it really hard to figure out what is actually healed and what is over-healed too
the healing threat multipler is only in effect (the new one of 12.1) is on tanks in there tank specs too so old formulars still apply iinc so you should be generate a lot less aggro is perportion to tanks.
I think the days of healers actually worring about aggro is kinda out the door now with how much damage players are doing compared to the little amount we're getting hit. with this huge buff to tanks aggro in 12.1 tanking (the aggro side) should be much, much easier.
<EDIT: In response to my comment (paraphrased) that threat in BBs being less of an issue to me in smaller groups and 6 mans-largely because i don't trait heals- but that raid BBs may be different....>
you'll be surprised :P
aggro in BB's seems to be set patterns with enemies going for set allies. but if that ally soldier is dead they pick a player at the highest aggro, thats normally the minstrel proccing a heal to something. so while the enemie is just spawning the healers get targetted and focused A LOT! enemies arn't as easy to pull off too because of the weird aggro system in there so tanks NEED to force taunt or dps need to keep an eye on there healers to kill mobs on them.
BB's can snowball into fail because of this pretty quickly. in beta many times if the group got a little behind and let just a few allies die, the healers ended up pullings 10's and progressively more every pull, after a while and started camping players at spawn point.
although I've not checked raid BB's on live or 12.1 :P