I usually never bother with FoF on trash pulls, but iirc (correct me if I'm wrong someone) it does affect Flash's aggro so, if that's the case, popping it before SL would be optimal. Otherwise, pop it after your Flashes. It's likely you won't need to use FoF for trash tho, so it might be better to save it for snap aggro on a patrolling mob or if you need to burn certain trash faster. Just make sure it's up when you get to a boss and you're solid.