At the first car seat clinic I worked at, playing with a loose seat belt was the primary complaint of a grandmother who came through with a Maestro in booster mode (grandson not present). The child was quite obese and over the weight limit for the Maestro's harness, but was only barely or maybe just under 3 years old, and in booster mode she couldn't stop him from leaning forward and playing with the front passenger seat belt and wrapping it around himself however he could.
Another family at the same clinic came in and at the end of the check as I was going down the form and came to the "unused seat belt" section (remember, I was a new tech and I probably would have forgotten it if it weren't on the form; so I hadn't put the seat belt out of reach during the install), the parents gave me a look like "You can't be serious, that doesn't really happen." I stood there and explained to them how the ratcheting thing works and could choke a child, and they gave me that "yeah right" look... until they looked in their car where their preschool age son was sitting in his car seat gleefully wrapping the seat belt around his neck. We were standing right there and hadn't had our eyes off him for more than 30 seconds at the moment, and he hadn't engaged the ALR yet so he was never in any real danger, but they sure changed their tune really quickly. We all had a pretty good laugh about it and re-did the car seat with the buckle tucked safely out of the way.
I always tell parents that accidents are very rare, but because they're so easily preventable, why take the chance? I also tell the story about the kid at that car seat clinic and that seems to take them from "no way" to "now that I think about it, kids this age are weird and who's to say they won't play with the seat belt?"