YOu have several options.  First, on the density tab, you have the option to choose "force regular alignment of instances".  This might be what you're after.  Second, on the Scaling and orientation tab, down at the bottom you will see a rotation option.  The Z axis is the lateral axis.  It's like you standing on the ground and spinning around in circles.  If you want all your buildings lined up perfectly, make the "maximum rotation" slider to read "0". 

TIP  Since you will be experimenting with populating a terrain with buildings this is what I would do.  First off, create a boolean box and apply a simple color to it - nothing fancy - black, red, or high visibility green.  Save it as a VOB on your desktop.  Then select the terrain and populate it with the boolean box instead of the buildings.  First, it will render extremely fast because each instance only has 6 sides and only a simple color applied to it.  Second, it will quickly enable you to figure out what settings you will need in order to get the results you want.  Then you can remove the boolean box from you ecosystem and replace it with the buildings you want ; ^ )

G&C