"auto" is only supported with HTML5. Maybe we should only add it with $wgHtml5 although browsers will probably just discard it as an invalid value.
As far as I know it is harmless to browsers that don't understand it and no further action is required.
One possible issue would be a browser not understanding "auto" and falling back to LTR. Would be a minor issue though.
Validating :D
Causes bug 34432 in Chrome
Reverted in r111798.