...Although for this once, it would be plotwise significant to make Betazed somewhat inaccessible from Earth. Kirk could have known all about Bajorans and Cardassians and Bolians and Benzites and never given them a second thought. But if he had been aware of Betazoids, his attitude towards telepathy would have been quite different.
Also, the fall of Betazed to Dominion attack somehow opens the route to attacks against Andor, Tellar, Vulcan and ultimately Earth, so one might want to place it between those planets and Cardassia. Probably not directly between, though, or Starfleet would have been expecting an attack from that direction. So a good guess might then be "farther away than Vulcan, closer than Bajor". I don't think too many other planets in the Trek universe have had it narrowed down to such exact terms...
Timo Saloniemi