Don't Move Me Cheese... or....

[Taqwus]

It really, really depends. You can't afford to mass armies everywhere, unless you're Dead Ermor or the game's already over, and if your opponents have decent long-range summons/attack spells (Horrors, Ghost Riders, Earth Attack, Mind Hunt, Wind Ride come to mind) the odds of stopping cold a reasonably focused attack via just a standing defence are not too good. Likewise, flyers and remote attack spells make a mockery of border-only defenses.
It's good to have a reaction force of some kind if you can manage one, and to have your own spells to hit back during the magic phase before an incursion can move on. In addition, hiring mercenaries in the vicinity can provide some immediate local reinforcements.