If your aim is to cover your liabilities, I'd go ahead and get the release. That way, you are covered even if the said person makes a far flung claims. On the other hand, if it's barely recognizable, meaning it cannot be clearly identified and the person in the photo was incidental, not the main model, I personally wouldn't bother.

Ah... the picture helps. I bet, if these images made to a gallery, the "models" will bring friends and proudly point out that the image is his/hers. But, if the image made it to an auction and fetch 10 million dollars, he/she will claim they are entitled to compensations....

I'd get the release.