It depends on where you are in the app. If you're on the Players tab, when you buy players into the tournament the software will not try to automatically balance tables. If the Settings window is closed and you're buying players into the tournament from the Game window, it WILL automatically try to balance the tables. Prior to 2.6, buying players into the tournament from the Controls tab behaved the same way as buying players into the tournament from the Players tab (no auto balancing). However, I changed this in 2.6 to be like buying players into the tournament from the Game window (DOES auto balance).
If the Auto Seat checkbox is checked, you get a different behavior. In that case, the Player Movement dialog will open after you press OK to buy the players into the tournament no matter where you buy the players in (from the Players tab, the Controls tab, or the Game window). But it isn't actually balancing the tables - it's only randomly seating the players who just bought-in.
The gist is that when you're buying players in from the Players tab, the app considers this just part of configuring a tournament (even if the tournament is already running), and thus buying-in and seating are two separate configuration tasks. But when buying-in from the Game window, the app considers this more like part of running the tournament than configuring it, and thus it balances the tables automatically.
* Of course, all of this is subject to the "Automatically suggest player movement" preference being enabled on the Tables tab.