My used bike came with a 700x28 tire in front and a 700x23 tire in back. Should I leave the wider tire in front, or should I switch the tires? What benefits or drawbacks are there one way or the other? Is there any particular reason why the previous owner might have chosen non-matching tires? Is there any reason I should go out of my way to buy a new tire to make them match?

The found a little bit of information about [tire sizing](http://www.sheldonbrown.com/tires.html#mixing), but I still don't feel like I know what to do.