I would say, trailing flat - never. Except for the winter.
For 5 years I towed the Hawk tilted - I never took it off, but I have certainly been pushed around a bit in over 16 knots. Also on the motorway when passing trucks, you feel the gust when you get level with the front of the cab.
There was a story of a tilted Hawk that flipped when leaving Grafham after an event a few years ago. I heard afterwards that it was blowing 40 knots and that there was a cover on the cat as well, so lots of windage.