There are instances, when it's acceptable but they're fairly few and between. One such situation is passing a guard on a healer while Kephess' red circle targets them, to reduce the damage they take by 5%.
That's a different reasoning though. Many tanks in FP queues think that guarding a healer will make monsters aggro them less.
The only time anyone ever guards me is during the 2nd phase of TFB. And even then, it's only for the first 30 seconds of that phase.