A lot of people say that the Acu-Wedge doesn't increase accuracy, but consider this:
1. You are squeezing the trigger.
2. As the trigger breaks, the loose upper shifts (ever so slightly), shifting the point of aim AND the point of impact. This means LARGER GROUPS.
Now, if the upper were bolted to a bench, like a free-rifle, it wouldn't make a difference. But a loose upper will shift the point of aim and impact if it's loose enough. As an extreme example, wouldn't a SUPER WIGGLY upper/lower combination have much more visible effects on accuracy? If so, then a slightly loose upper/receiver might have a slight impact on accuracy.
All that said, the Acu-Wedge is a must, because a loose upper/lower means REDUCED RELIABILITY in feeding. I've carried M16s that were so loose that they wouldn't feed a single round, and I had to tape the upper and lower together just to get it to work.