When I got my first free boost, I ask everyone I know this question "when to boost".
The answer is not matter too.
Both the growth and the current ability will be boosted by 20%. Just use it ASAP to enjoy the boost. lol
Highly recommend boost ur Elite Engine first if you are a BB player. I am kinda of mean, so I save my boosts till my brought some Elite Eng/Rep as I want to max the advantage.
Enjoy you boost! :D