That is the whole idea. Defer the charging until you actually need to have the engine running. In an ideal situation (for maximum fuel economy), you would only have two states. The first state is with the engine running at an efficient state propelling the vehicle and charging the battery as necessary. This would either occur during acceleration or when going up a hill. The second state is coasting with no charging or assisting. This would either occur during deceleration or going down a hill.

Of course this is not practical in most people's normal driving environments, but it is good to know. If you are aware of how to get maximum fuel economy, you can extract what is applicable and apply it to your daily commute.