Assuming you're talking about translating logic levels- if you're not too concerned about speed or power consumption, the resistors will work.
However, assume something like 30pF of input and stray capacitance, and resistors that equal (say) 10K in parallel and 40K in series (2 20K resistors). The resistors will add a delay of around 300 nsec and will consume 90uA for each that is high.
If you're talking about the power supply for an IO port, you should use a regulator. The regulator will draw less current and will provide a stable "stiff" voltage (preventing port pins from interacting with each other, for example).