It's got something to do with a disease called Aster Yellows, but apparently many people do plant them in the same bed, and then stop only if they get the disease. Makes sense to me :)

If that were the case we'd all have to move our perennials like Echinacea, Rudbeckia and other prone plants around every year. NOT! LOL!
I have annual Aster Crego that has re-seeded in the same area for 5 years. Also know it's not just Aster's or Asteraceae family members that are suseptible to the virus. You got Cockscomb, Mums, Petunia, Marigolds, Dianthus, Coreopsis (annual and perennial), Glads, and certain veggies that can become infected.

Vera

yep, if all are healthy, why worry. My favorite adage: if it ain't broken, don't fix it.