In a large game, you should expand quickly, than fortify everything you have captured and then develop your economy
FYI - Fortifying only works in MP if the planet is a choke point and is hard to go around. Also, turrets are generally useless in 1.05/1.11 as you can go around their area of effect. You need hangers since the fighters/bombers can go anywhere in the grav well. Also, repair bays are important to keep said hangers and other structures (e.g. frig factories) from going down quickly when being attacked.
At the end of the day, the best way to defend a planet is either
1) Have a fleet there to repel the attack and/or
2) attack your enemy so they are defending (best defence is a good offence!!!!).
However, on those rare occasions that you have a planet next to another player's planet (like me last night), and you KNOW that one of you will have to attack the other (due to map layout, plus the fact that if one moves, the other will blow through), then you want lots of repair bays where the enemy will jump in and your fleet there waiting. It will keep your ships alive a bit longer. And a PJI on the far side of the grav well would be good to make it hard for them to flee... PJI plus Vassari egg is nasty when trying to retreat!!!