I have saved the game before battles and tried autoresolving compared to fighting the same battle out about 6 times now on vh/vh. If you have better than 3:1 advantage it doesn't seem to matter except you will probably not inflict as many casualties with Auto. If it is close you will do much better fighting it out yourself as opposed to Auto. I have had crushing defeats on Auto that I was able to win myself. I think Auto under rates the value of missile units like horse archers, that can decimate spearmen and militia, particularly with the passivity of those units under fire. (I think this will change with a patch?).

As far as whether it is cheating or not I don't see AR as such. Cheating is something you do to gain an advantage. AR just saves time but is likely not to give you the same advantage as fighting it out.