All I do is F-Air...and it's not exactly retreating most of the time. What is the exact use of N-Air that F-Air can't do? And should I also commonly use B-Air more for retreating? I tend to sort of save it for killing considering how damn strong it is for such a fast move.
First things first, it's not bad to throw out the Fair
most of the time. It's the best spacing tool Ike has, unless you count a perfectly spaced auto-canceled Nair. The biggest issue with Fairs and Nairs is shield-grab potential if you get too close (if you are getting too close, you're not spacing right). Hitting with just the barest edge of the hitbox is hard but entirely possible.
Nair has less landing lag and leads immediately into jab1, which is out on frame 4. Nair is an all-around awesome move, just not so much in the IC's match-up. Since it covers most all of Ike, it's good for covering landings instead of airdodging into the ground. Note that this isn't always the case, as someone with a fast character like Fox can easily wait for you to land with Nair and punish with a sliding Usmash.
Bair isn't really effective OoS against IC's because of their height. If they are in the air, however, feel welcome to use it. Bair is plenty fast enough to avoid punishment, at least as far as aerials go. When fighting around Mario's height or taller, OoS turn-around Bair is usually a very good option. Don't be afraid to let it stale a bit. That's why Ike has pummel, jab, and Nair.