Upgrade heroes at Town Hall 14 to maximize 12-day upgrade timers and get the best use out of books and hammers.
For players at lower Town Hall levels, upgrade early levels of heroes to make the best use of dark elixir.
During clan wars, upgrade Barbarian King, Archer Queen, and Royal Champion to lower levels to minimize downtime.
Utilize builder potions and short upgrade times to upgrade heroes without losing income from wars.
At Town Hall 16, go all-in on hero upgrades, especially if you have Max sets of equipment.