smj0730,

Topping is not necessary. In nature, the plant wouild grow without being pinched back. If you wish to show in a garden competition, then you would want nice tall straight stems, so you wouldn't "top" at all.

However, if you are not showing, and wish more flowers, albeit a little later, then you can pinch back the stem to force it to produce more shoots out the side. This will create a bushier plant. I do this here, as I do not show, but prefer more flowers. So when it gets taller, I pinch back to above several true leaves. We are in a completely different growing zone than you, so I feel confident in doing this here early in the season as the plant is growing. In your zone and if your plant is about 2.5 feet tall, you may have hollow stems already in the lower part of the plant. I don't pinch back that low as I don't want water getting into the hollow stems and rotting the stem / plant.

Pinching back takes place early in the growing season. As I see it, it really is a means of shaping the plant in its early stages to a shape more to your preference. I hope that helps. Other growers may have differing opinions as well.