Better to go for magnetically shielded speakers if you can (I think most mainstream manufacturers do this as a matter of course now, due to so many people using speakers for AV duties). Even cheap speakers will now be shielded, in fact, they may be more likely to be shielded than the very expensive 'hi fi only' speakers.
If you really can't get shielded speakers, 0.5m should be a minimum distance away from the TV. Ideally from a sonic point of view and for the sake of your tube, 1m away would be better.