RE: Tire replacement
If youre talking about a car with a short sidewall tire (smaller car or smaller rims), I'd replace it to match the other one you recently replaced.
If you stagger the tires with tread depth, not only will you put more stress on the "worn" side components (tie rod end, ball joint, etc) but youre taking your alignment away and wearing the new tire excessively and unevenly.
If youre talking about a large car or pickemup truck with tall sidewalls and larger diameter rims, it wouldnt be as detrimental to the vehicles steering and suspension and Id say the decision wasnt as critical.
Of course you could rotate the tires to make sure the worn one was in the rear and it wouldnt matter much at all. LOL!
I did assume it was on the front, didnt I???
[X(]