I haven't sown my zinnias yet this year. I've always grown rows as a bulk seed drop with good luck even though it always says to thin them on the packets with a "survival of the fittest" mindset. This method doesn't facilitate thinning very easily.
Last year I painstakingly dropped individual seeds about an inch apart and although it made a nice, full display cutting drastically changed the overall appearance for a few days. I think I'll go back to my older method again.
One great use for the tall varieties is to plant a row at the front of the veggie garden to hide the peak season weeds from most visitors!