Perhaps I'm the only one who feels this way... but if you have an opponent seriously glassing your planets, I consider it too late.
In other words, your defense should not be near your planets, but at warp points leading into your space. If the enemy fleets get past _that_, then you're screwed, but you would have been just as screwed if that large choke defense had been split up defending planets.
There are several exceptions to this rule:
1. Early game (<~20 turns). A few WPs can take out any rush fleet. Or should be able to.
2. Very late endgame. If your opponent is opening warp points into your system, you have othe rproblems on your hands. The best defense here is not to let your enemy survive that long
LL