If a tyre abruptly deflates, the vehicle will be more controlled if it's a rear tyre, I believe. In terms of putting the best tyres on the back, I attempt to change all four at the same time. Assuming no damage necessitates replacement, this implies half the miles will be covered with the poorest rear tyres. I switched Mrs F's Pug's tyres front to back after 15,000 miles, assuming a 30K mile tyre life. There is just a centimetre difference, and as more miles is accumulated, the difference will diminish until they are equally worn when replacements are necessary.
This concludes some more 'normal crap'.