In my experience, it depends on a few factors:
> Down here where I am (SW CO), we run a lot of sandy, silty rivers and the vee that is formed by the tube and the box catches grit, and it works the tube. In cleaner NW rivers, it may not be a big deal.
> As I learned the hard way, it's especially bad with an Aire boat, cuz the zipper seam is right on the rub point, and in about 2 San Juan trips, it went right through the sewn thread in the zipper seam (in a couple of small spots), so I repaired that, then glued a chafing patch over that whole area. And mine doesn't even rub hard, it just touches.
> I think my rule of thumb has become: I want a gap.