Here's a quick tip I discovered in the Apple Store the other day: the crop functionality in the Photos app will also allow you to tilt photos with two fingers. This is different from the 90-degree rotations available from the dedicated rotate button because you can simply tilt your photo by degrees. Does that sunset look just a little off? One quick tilt later and it's totally level.
So if rotating is overkill and plain old cropping just won't do the job, remember that you can tilt straight from the Photos app. Just tap on the photo you want to adjust, tap Edit in the top-right corner, top “crop” in the bottom-right corner, and use two fingers to adjust the image to your liking.
[hat tip to the repeating Apple Store tutorials that I had to watch while waiting in line for the Genius Bar]