Most of the time, it's not worth the walk to the F check-in.

The only time it's convenient is when the taxi driver manages to drop me off right at the end of T5. Sometimes it's not worth the risk of having to hang around and wait for a space if there is a space before that zone in case they are delayed so I just say drop me off where-ever if it's busy. I generally have them drop me off at the area that's fairly close to South Security as I'm too lazy to bother walking. F check-in requires a bit of a walk to South security, so for a lazy person like me, F check-in is not so good.

Of course if all other check-in zones have much bigger queues, then it becomes worthwhile but otherwise I'd use any other nearest, relatively quiet desks (including ET/WT ones), weighing up the walk afterwards to the security screening area.