Hmmm. I think that by the time you can give a "release as a test" it will no longer be a reward, because your horse won't need or even understand it as a reward. That is, he will by this time be comfortable and seeking the contact of the bit; so what's good about losing it?
The test, in my understanding, is to see that he is stretching and the energy is going through from his hind. A horse who is heavy in front will be leaning on the rein somewhat, so if his head moves outwards when you release it, he is probably not balancing well.
So when do you use it as a reward? I had a hard time getting my horse to accept the bit, and I would "give away" the rein whenever she would finally relax her head and jaw; and yes, the contact was stronger then. Eventually the hold got lighter, and the release lighter. But it wasn't until I finally got a good lesson that I discovered I was giving away too much, that she didn't need the "reward" any longer-- being quiet and riding with only the seat WAS the "reward."