I only started riding a recumbent bike last November - a Performer. Initially, I fitted a flag for my rides around the rural lanes around my home. However, it was soon apparent that flags upset horses and I took it down. I don't need a tonne of steel-shod hooves trying to sit on my seat when I'm already in residence.
So I took it down. I've always found, having covered well over 1000 miles, that vehicles give you plenty of room. Discussions elsewhere have concluded the same, largely because (it is thought) that drivers spot something odd in front and slow down and give you a wider berth than they might for a DF bike.
I have not yet had a close pass on the recumbent, but I have had a few on the DF - two this week!
I've not used a flag for months and I've had no problems - indeed I get noticed because when I mention to people in the village that I also ride a 'recumbent' and explain what it is lots of them have seen me on the road. So, the vision sticks.