I wont push this point, but just to share a thought... One benefit of a dedicated folder is that it "advertises" the availability of a OpenBSD. It helps potential users "feel" more confident about trying it. There will be a percentage of people who came looking, didn't see any reference and just moved on without asking.
If you don't want separate folders for Free/Net/Open-BSD, perhaps a **build.bsd64x64** folder would be appropriate?