The trick is to plant as closely as you can to make the most of your space. However, the trade off is that the closer you plant, the more likely you will experience disease and normal die off.
I think it was Beyfuss who said that no matter how closely you plant, the die off will continue to the rate of about 5 seeds per square foot. Myself, I see 5/sq ft as about 6\" spacing between plants.
The idea is to use the least seed possible while getting the thickest planting reasonable.