My question is: Is it wise or good practice to use a voltage divider on the AC side of the bridge to knock down the voltage so the 7805 doesn't have to get rid of so much?
No. Voltage dividers don't work very well for power, because they waste power and the voltage drop varies with load impedance/current.
However, you could use a 7812 (12V) along with the Arduino's on-board regulator.
With the approximate 1.4 times voltage "boost" you get from the rectified 24VRMS, you'll be coming close to the 35 or 40V maximum rating on the 7805. So, make sure to check the specs on your particular regulator and measure the rectified DC voltage.