A voltage divider is not risky because the supply is not stable, but because the load is not stable. Voltage dividers CANNOT provide adjusted voltage for anything but the tiniest loads (usually caused by high-impedance inputs).
For bigger loads, that is, small effective resistance of the load, you will get significantly different effective voltage division, since the resistor to ground is in parallel with the load resistance. Imagine you set up a voltage divider from 12V to 5V with a 350 Ohm resistor and a 250 Ohm resistor. First, this will constantly drain your battery with 20mA. What happens now, if you add an Arduino drawing 20mA at 5V? It will have an effective resistance of 250 Ohm. Now, your resistance against ground 1/(1/250 + 1/250)=125 Ohm. So your Arduinos HIGH will be at 3.16 Volts, which is not enough for a 16MHz Arduino. with a lower resistance voltage devider, this could be somewhat countered, but at the expense of higher constant power drain. Remember, you are already at 50% efficiency in my nonfunctional example.
Since LDOs like the 7805 just burn excess voltage in heat (according to I*diffV), they are also not a great choice for large voltage reduction.
Use a switching regulator. The MP1584 is good and cheap.