The northern building thing isn't really correct..
First of all, it's the northern unit or building. Doesn't matter which it is.
Secondly, this only applies to airstrikes and units set to Hunt mode.
If you take superweapons, for example, they do a threat assesment. Nod will always use the nuke on either the Advanced comm center or the base defenses. GDI will target the ion cannon at base defenses, in this order: Obelisk, SAM, turret.
This means you can outwit the Ion Cannon by building one SAM site as decoy (it doesn't get destroyed by the ion cannon unless it's opened) and using blocks of 4 turrets instead of building Obelisks. Sure, it costs more, but it kills anything
Oh, and note that sandbagging doesn't work in RA1.. the units destroy any walls in their way. And you can't build structures on sandbags in RA1 either.