It's mostly by studio. Randy Blue is all condom, Sean Cody is all bareback and does quick tests/certifications for HIV, Treasure Island Media is all bareback and owner Paul Morris has a strong political/social stance against vilification of those with HIV. Dallas Reeves' studio is bareback, but is getting some flack from a model who claims Reeves is hiding his HIV status. Reeves may lose models and business from it, and once you work for Paul Morris, you'd better hope he has a contract for you because you're pretty much blacklisted from other studios.
There's no small amount of debate between the studios/producers about this divide in philosophy. In fact, the conversation as it plays out in the adult gay media resembles the last several years in the U.S. legislature.
This interview with Morris gives you a good idea of the personal convictions and lines drawn in the sand:
A Porn Director Stirred Up Controversy by Making a Movie Centered Around HIV | VICE | United States