Using resistive dividers in sampling systems in this configuration (involving a clocked ADC) are not advisable. The resistors will be producing white noise at your input. The problem is, when your system samples this noise voltage, the noise power in the whole frequency spectrum will appear in your base band, and degrade your measurement. The solution to this is to add a low pass RC network (a parallel capacitor to R2), which will filter the high frequency components, limiting the noise (and the signal) frequencies to your desired band of operation. The time constant of this RC network should be dictated by the frequency of the input signal.
Also, since you're going to be scaling your input signal with R2/(R2+R1), the matching of these resistor values are somewhat important as well. Otherwise, you're going to be making a linear error, which you can later correct digitally. There are other problems as well (nonlinear resistance changes with current or temperature), however, considering you're going to be using the Arduino ADC which should be in the range of 10-12 bits, I doubt this will pose a noticeable inaccuracy in your case.
