|Would it be worthwhile to pinch off the growing end of a Mexican Sunflower to make it fuller?|
|If you pinch seedlings while they are still very young, before they've put out any side branches at all, you risk removing the only growing point the plant has. Wait until side buds are visible before pinching back any seedling.
By the way, Mexican Sunflower (Tithonia) tends to branch just fine on its own without any pinching. In fact, they can get HUGE!