Milln found something similar to this for Lucario, and basically, if you hold the stick at a non-notch position when doing Fire Fox and right when he "blasts off," you slide the C-stick from a perpendicular notch to the nearest notch to what you were at, you'll curve it automatically and most often, it'll be the perfect curve.
Hard to imagine, but just go to Training Mode, and do Fire Fox. Hold Up-Up-Left until he fires. Immediately after he fires, let go of the control stick and slide the stick from the left-notch to the vertical notch quickly while Fox is moving. You'll do a perfect curve.
This might be an easier way to curve it for some people. Just throwing this out there if it wasn't known.
Hard to imagine, but just go to Training Mode, and do Fire Fox. Hold Up-Up-Left until he fires. Immediately after he fires, let go of the control stick and slide the stick from the left-notch to the vertical notch quickly while Fox is moving. You'll do a perfect curve.
This might be an easier way to curve it for some people. Just throwing this out there if it wasn't known.