Originally Posted by jsloan

Not necessarily. There is a huge advantage to booking RTs if you expect to change the return flight and might need to pay a fare difference -- you get to calculate advance purchase from the date of the original purchase. So, if you have a RT G fare, there is a K fare in the market (say, +$20 each way), and you need to change the return, inventory is K1 G0, and the 0-advance purchase fare is an E (+$400 each way), booking an RT will save you $380 on this change. The numbers are made up, but the scenario isn't -- I've been on both sides of it (a RT, and a cheap change, and two one-ways, and an expensive change).