This one has always been a hot topic and I've been fairly opinionated on this in the past, but today there are so many published ways to do this I thought I'd share a simple way of automating the removal of the AT status as you work:

;------------------------------------------------------------------------------
; Clear AT statuses in SDL Trados Studio.
;------------------------------------------------------------------------------
^+f::
Send ^a^c!{Ins}^a^v
Return

The script is simple enough, you just press Ctrl+Shift+f and the AT status from a machine translation magically disappears so the active segment appears to be a new translation for you to post-edit.  The script just automates the selection of the target, copy the target to the clipboard, overwrite the target with the source which clears the AT status, select the target again and paste back the original machine translation.