When GW releases a battletome for a faction, they update the bases in the boxes for that faction. So, some still have square bases, and some have round bases. So, unless it has a battletome, odds are it's still on squares.
Fortunately, it doesn't really effect the game all that much. Besides the fact that square bases in a skirmish game look like amphetamine parrot. All models look better on circle bases. Also, GW has their own bases you can buy at extremely aggressive prices.