If you rotate the tires as recommended, they should be replaced in sets not pairs.
Most people don't replace worn stock tires in pairs because 1) stock tires aren't good performance wise so nobody wants to pay for a new set and 2) running stickier/better performing tires on one axel and not on the other is going to give the car unpredictable handling characteristics, or 3) they actually rotated their tires as recommended.
If you've worn out the front tires, you CAN just replace the pair. However if you put summer tires up front and stock all seasons in back you may find your rear not getting good traction in tight turns. Depends on what tires you're currently running though. Stockers are not good for traction in any conditionss really so buying 2 new stocks is a waste of $$$ if you like performance.
Granted, after I lowered my RSX, I burned a through a pair of stock michelins in few months due to poor alignment, but after everything was set, all 4 tires were replaced.