With TinyMCE, Shift+Tab would generate a tab character. Redactor doesn't do this.
Interestingly, if you have moderator permissions then actually with TinyMCE Shift+Tab would select the previous inline-mod checkbox so this user experience would be inconsistent depending on your permissions.
But if there was a suitable key combination (Such as Shift+Tab but ensure that it can't be used to tab focus backwards to a checkbox) then that would be ideal.
Not sure whether the Shift+Tab thing was just a Chrome thing or a feature in TinyMCE.
It's definitely useful to have some sort of tab character. I know white space is preserved now, and 4-5 spaces would achieve a similar result, but some other shortcut would be perfect.