When I last shopped tubes at the Scout shop, I went in with the expectation that 2,3,6(?)*, and 15 were the best consistently. Simple spinning on an axle showed that the bores were centered and even in the 2s, 3s, and 15s, and that's what we wound up building our cars with.
If you're getting wheels from John, the numbers become unimportant as he has worked the wheels and made them all good.
If you're buying from the Scout shop, for a scout build, I would definitely seek out and create tubes of 2s, 3s, 8s, and/or 15s*.
*Randy from Maximum Velocity tested a random sampling of wheels at least twice now. The first time (2010?) his tests came up with 2, 3, 6(?), 12(?) and 15 as the best molds (I don't remember the specifics any more). He
ran the test again in June 2014, and found that 2, 3, 8, and 15 were the best.
I don't know (and doubt) that the Revell wheels follow the same suit, so YMMV there. (Plus, you can't crack open those tubes and swap wheels around to create your own tube)