I think it is really personal preference.
Changing one saves money.
Changing two ensure good rubber, and less frequent trips for tire changes.
If I changed my own tires, I might do the one at a time thing, but since I don't, I change them both at the same time.
I don't want to change one, then go back in a month or two and change the other. Besides, even though the front wears longer, they usually start to feather and I don't like the ride as well as a new one.
It really boils down to a money / convienence type of decision.
2008 BMW R1200RT 7200 miles
2005 BMW R1200GS 13000 miles (Sold)
2002 Yamaha FZ1 (Sold) 11000 miles
2002 BMW K1200LTC 29900 miles (Sold)
2002 Yamaha VSTAR-650 Silverado (Sold) 9100 miles