In short, a properly spined arrow will stabilize in flight MUCH quicker than an improperly spined one. I know you're shooting mechanicals, but when you screw on a fixed blade head to an arrow that is already working hard to stabilize itself, well....It's amplified tremendously.

Now, will that matter much at 20 yards? Like Bob said, probably not if it has time to stabilize itself. If the arrow is still stabilizing itself when it hits said target, it won't hit squarely, and KE will be absorbed by the arrow instead of transferred to the BH, thus affecting penetration.

We all know a properly tuned bow is the most accurate, and efficient. Properly spined arrows are part of that equation. But as has been said, it's better to be over than under. If you're under, your arrow is burning up the KE by flexing.

I know what I'm trying to say darnint, but maybe I'm not saying it very well....