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.
James Ranks
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