FSF recommendation is that you use the same license as the project which you are contributing to. If you use a BSD project, contribute your patches under BSD. If its GPL, contribute under GPL. If you combine work under BSD and GPL and write modifications, contribute back the modifications based on what code you are doing modification for.
The proprietary way is to release modification only if its make a business sense to do so, and I don't know if OpenBSD actually has a official recommendation in this aspect. If they do, its not something I have ever seen.