I don't use a flag on my Streetmachine because it's the same height as a car and I've never felt the need.
Barakta has one on her trike, mainly to serve as an aerial for her radio aid. I consider it a mild eye-poking hazard that also serves to spook the few horses that aren't already bothered by a low recumbent. I don't think anyone has trouble spotting recumbent trikes on the road, and when I ride it, I don't usually bother.
I don't use a flag on the Red Baron, because it isn't in the spirit of aerodynamic gains. I'm aware of occasionally disappearing behind a motorist's left wing as they overtake (you can tell because they give you even more room). I don't like riding the Baron in urban traffic more than necessary, mainly because of the low tolerance for potholes that drivers are completely unsympathetic to, but also because being low down and highly reclined limits what *I* can see. It's not unsafe, but the extra caution required becomes tedious.
I don't use a flag on my Brompton, which does have serious being seen issues, because I don't think it would help.
From general observation, most recumbent flags do very little to improve visibility from behind, because the air-stream renders them 1-dimensional. To solve this I think you need a windsock or turbine blade design, increasing the drag.