I tend to buy them 4 at a time, once the fronts are just over half way through their life, I swap them to the back and then all four wear out simultaneously. I find you can negotiate/find better deals when buying a full set.
If I was just changing two, it would be entirely dependent on how much thread was on the back tyres. If it was anything less than 2.5~3mm, I'd swap the worn one's to the front and get ready to spend out again in a few months.