Another reason I prefer to run an earth cable from battery to battery is that you eliminate possible fault points.

If you run an earth to the chassis at the front and then another one from the rear to your power supply there are two possible problems down the track.

A loose connection or the connection getting dirty, things might still work but not at 100%
and if you decide to later add a battery to the rear, a bad earth could mean not charging properly, unbeknown to you.