Part of the answer depends on your particular gearing. If you look at the ratios for your chainrings/cassette combination, there typically will be a logical crossover point.

This is a good graphic aid: http://www.gear-calculator.com/#

As other have mentioned, when you shift in the front, you also typically want to shift a couple of cogs on the back, to prevent making a big jump in the ratio. This is particularly true on a compact, given the wider difference that is typical between the chainrings (i.e.16 teeth instead of 13)