With annuals like zinnias and cosmos (both of which I plant a lot), I tend to sow them pretty close and don't worry much about distance initially. Both are easy to move around when seedlings emerge. Ultimally, I leave 12 to 18 inches between them, I suppose. Zinnias can get mildew if they're packed too closely, but that's not a huge issue in my climate. I always space annuals more closely than the package recommends. That's my 2 cents worth anyway.

dan