In order to close an oversize seat tube on an undersize post. the clamp bolt
actually BENDS the wall. Three things could make this bad: 1.) If the
slot at the top of the seat tube is too narrow, the edges would meet, and
further tightening would be ineffective and possibly harmful. 2.) If the
bending stresses exceed the yield point of the seat post material, the tube
would take a permanent set, and it might be hard to insert a larger tube
later, as previously mentioned. However, I think this is exceedingly
unlikely, since the amount of bending is very slight, and bikes are
generally made of high yield materials. 3.) If the oversize seat tube is
snugly tightened at the top, there will still be clearance along the lower
portions of the seat post. This could conceivably produce a small amount of
wobble, which could be annoying, and might possibly cause the post to loosen
after some miles of use.
I have done this, and had none of the three problems I just mentioned. I
suggest you try it and see.