Lets not compare apples to oranges here. No pun intended.
The way some people, meade included, is the right way. Others, who just use the headphone jack do it the "wrong" way.
Doing it the right way includes giving the radio the ability to go into the ipod,ipad,iphone's database and read and convert the song to audio.
The wrong way uses the Aux cable and just lets the radio play amplify whatever it is told to, already converted using an inferior converter. This way does not CREATE distortion, but it greatly increases the chances.
Remember, if you use a bad signal to begin with, no matter what you do you'll end with a bad signal.
Now, your thinking" well what if i cannot give the radio access to the iproduct's hard drive. Fret not, just keep the volume below (estimate here) 7/8ths of maximum. That will decrease chances of distortion enough for the average listener.