i felt like my loops were better when doing this too. this a coincidence or good form?
It is not coincidence. When casting directly overhead, you eliminate all lateral miscalculations. For instance if your trajectory is a little off, your distance may be a little short or long, but it will be in a straight line to your target.
If casting on a 45 degree angle, distance will also be off, you will not be in a straight line to the target either. You will miss either right or left as well.
Basically,wiith direct overhead casting you have "removed" half the possibility for error. That is an oversimplification of course.