Pinch early, pinch often and your zinnias get bushy with lots of flowers.
If you don't pinch and they get leggy, you can bend them horizontal and the branches will grow vertical, also effectively giving you more flowering stems.
I only bother growing the "giants". Anyways, if you have more than one zinnia, pinch one hard, leave one alone, you know, experiment to see which technique you like best.
Zinnias are the best annual flower, period.