In 2009 you need to right-click on the 'extend baseboard' button in the terrain tools tab (the one that lets you add new sections of baseboard to the route). A small panel will pop up in the center of the screen asking you to select if you want to use either a 10m or 5m grid for your baseboards. After that, any baseboard sections you add to the route will use whatever grid type is currently selected from that panel.
You can also convert any existing 10m grid baseboards (for example, from older Pre-2009 routes) to the new 5m grid by clicking on the baseboard you wish to convert after selecting the 5m grid mode as described above (note that you can't revert back to a 10m grid once you do this, so make absolutely sure you want to change it to a 5m grid.)
On a side note, despite having the newer 5m grid available, TS2009 will use the 10m grid as default on the initial first base board when you create a new route.
