So while there isn't a particular reason to cast Pyro right away when IB is on cd, there is a reason to hold it at least 2-3 casts, because if any of these casts crit, you'll have an increased chance of turning that HU into a Pyro!.
If IB is off cd and you have a Pyro!: use it right away to make room for a possible HU.
If IB is on cd and you have a Pyro! + HU: use Fireball + Pyro! for an increased chance of making HU become a new Pyro!
If IB is on cd and you only have a Pyro!: save the Pyro! a few seconds for a chance of getting into the above situation
Just make sure you avoid getting into a "IB is off cd and you have a Pyro! +HU".
You don' t have to avoid getting into a "IB is off cd and you have a Pyro! +HU"
Casting your pyro! and your IB just after. Thx to the delay of pyro, you'll get a new Pyro! from previous HU +IB