Funny, I was just having this conversation today.
Whenever I get a codex, I boil everything down to the "best" configuration. Then I end up spamming that configuration of unit. It is always inefficient, but usually devastating to one type of enemy unit or another. Then I find out what the 'comfortable' numbers of that particular config I need, then I'll incorporate an alt config for additional iterations of the unit.
This has led me to slightly rethink the hull heavy flamer. On my mechvets, it is still hull heavy flamer all the way. They are constantly moving for both offensive and defensive reasons. their weapons all have an effecitve range of 12" and they are either shooting land raiders or hard hitting heavy infantry. Alternately, they are maneuvering towards an objective, one that might be claimed by stealthed troops choices.
The same goes for my company command squads. they are packed to the gills with meltas or plasmas and are usually sent on near suicidal missions, always supporting my mechvets. they get hull heavy flamers as well.
That gives me around 5 hull heavy flamers, which in my experience has been ample, especially considering I've been running two banewolves with hull heavy flamers as well. Heavy bolters can reach out and work on enemy gunlines or armies that don't have an assault threat to your vehicles, and I'm fairly certain my psyker battle squad chimera will end up with a heavy bolter very soon.
Also, in infantry platoon based mech armies, like darkwynn's leafblower, you are often firing a single heavy weapon out of the top of a chimera, in that case a heavy bolter will come into its own more often, and you will have ample infantry units to screen
PCS and infantry chimeras from charges, they won't block
LOS so the low angled heavy bolter will have open fields of fire to chatter away.
As always, my musings are subject to change after my butt gets kicked, but here is my new chimera equipment list.
CCS 4x special weapons gets heavy flamer
veterans with 3x specials and no heavy get heavy flamer
PCS 4x flamers gets a heavy flamer
infantry squads with heavy weapon get a heavy bolter
PCS with heavy weapon get a heavy bolter
PBS get a heavy bolter
storm troopers get a heavy flamer (scouting heavy flamer vehicle should be obvious)
As for the artillery, it is much more simple, and I've been sticking with the same rules of thumb since april.
hydra gets heavy bolter (it doesn't want to move anyway, and fires long range anti-infantry/light vehicle)
manticore gets heavy flamer (can't fire both if stationary, only loses the indirect option if it moves, deters assaults)
griffon gets heavy bolter (can't move and fire, if you've got
LOS the heavy bolter can contribute)
medusa gets a heavy flamer (big blast is ordnance so can't be fired with another weapon, small blast is so focused anti-tank, that adding a heavy flamer gives you more flexibility)
colossus (depends on if its ordnance or not, I don't remember, if it isn't ordnance, then see griffon, if it is.. see basilisk)
basilisk gets a heavy flamer (ordnance shot means you won't be firing it in addition to the heavy bolter, so might as well deter assaults)
As for leman russes, I have 3 of each turret, and then three of each of these configurations (which i find to be the only logical configs)
single heavy bolter
triple heavy bolter
single lascannon
heavy bolter/plasma cannons
lascannon/multi-meltas (barely makes my list. Really just there in case I write some list with no melta, and for apocalypse with a vanquisher turret)
I contemplated single heavy flamer for a long time as a viable option. But for most of the russes, you really want to stay way back. they are almost unkillable from long range. But get frighteningly fragile (especially in squadron form) up close. Thanks to lumbering behemoth, there are plenty of opportunities to fire off that heavy bolter, which increases your overall long range output. Demolishers and punishers really want to get in close, but I've found that while running them, I tended to want sponsons, and once you've splurged on sponsons, you won't ever consider moving to get a turret shot and a heavy flamer, when you can rotate, fire two plasmas and a heavy bolter or three heavy bolters and the turret.
hope that was helpful, if a bit long