I shield into fortress most of the time when people are coming at me from above, but I shield grab every dash attack. It's easy to realize if the person you are fighting is hitting all of his Lcancels though, and if he isn't, then I shield grab when someone is coming down with an aerial. At FC I fortressed almost every aerial, but grabbed everyone who dash attacks... like the peaches, foxes, falcos, you get the idea. I fortress everytime I know I will get a killing blow out of shielding though, whether it's aerial or dash attack.
Another thing... I don't like the fortress unless I am shielding into it or retreating with it. There are times when you do it, and you try to nip the opponent with the end of it for 3 percent, and then because it's so quick you can get it off again, but it's rare and almost lucky. I never just strictly fortress going right into someone as my way of approaching them or getting on top of them. A good player with someone who has a great "Bowser Grab" will destroy you. What I call a Bowser Grab are the few characters who can grab you out of the fortress easily... Peach, Sheik, Marth, a couple others. My friend who plays Peach grabs me out of the fortress 90 percent of the time, whether I'm coming back from off the level, or doing it on the ground. He never dash attacks against me either, he just runs up and grabs, like I do to people. I'm convinced if you use peach you should never dash attack cuz you can run up and grab, then forward throw and make a combo out of it - at low percentages of course. But almost everyone who plays this game shields when someone approaches them, so just grab, get the move off, take what you can get, and then go from there.
So yes, shield fortress when approached from air, shield grab dash attacks, shield fortress any attack from high percentages.
And the fortress isn't all that good except in that situation, and retreating away from your opponent. And coming back from the ledge of course. It's a good move, not super great though.
Luke