I only build defenses on planets where I have important production facilities, like shipyards, training facilities or contruction yards. For those planets, I put 2 gencores and either 9 Sullustan or 3 Dark Trooper regiments, to prevent sabotage. That's it. The rest gets nothing. Sure the AI comes along now and then to bomb or sometimes assault the planet, but I keep a decent sized fleet in each sector and just bomb those poor bastards the AI dropped on my planet. This helps keep maximum maintenance free for attack fleets. I used to play super defensive, now I play super offensive.