I would say that is an added precaution to earth the rear covers of panels too. Generally we do not do this as rear covers are screwed and as such while we access the rear covers, the panel would be made dead and hence no danger. Whereas front cover access can be when the panle is live too. Moreover, many a times the front panel would have on them live components such a spush buttos, indicationg lamps, selector switches, meters, etc. thus waranting a dedicated earth connection. As the rear doors would not have any such arrangemnet, we do not generally earth the rear doors with a dedicated earth connection.