I almost exclusively use undersized tubes; a 700X18-25 tube is smaller than a 700X28-35 tube, not to mention easier to install (very little chance of it getting stuck under the bead). All my bikes have an undersized tube in their tool kit. It turns out that the only time I change my tube is when I get a flat on the side of the road. If I'm building up a wheel from scratch, I'll use the right size tube, but that's about it.

It hasn't given me a bit of trouble, not even the 700X18-25 tube in my 45mm hybrid tire.