For me, the biggest inhibitor from using Heavy Tanks is the shell count. Most of the time I spend going back to the depot to re-arm, instead of being in the action. Even if you have someone repairing you in the field, you will run out of ammo and that will take out of the action for a while.
This becomes more accentuated due to the slow speed of the Tanks.
Something like 24 starting rounds on Tiger and 30 on Churchill will be good.