I've had bounty hunters come up to me and have a conversation, and when Latula (my Sith) gently reminds them that they should follow proper Imperial protocol when addressing Sith, have refused to do so. A different sort of Sith might have Force-choked him for that. It's considered having proper manners.
Actually BHers are not part of the Sith Empire, they are merely being paid rather well for their services. They are not imperial citizens and thus are not subject to Imperial Protocol. BHers are known for not responding predictably to authoriy and it honestly makes perfect sense for a BHer player to rarely actually refer to a sith as "My Lord" or such because it is not inbred to the archetype.
Now I do not mean that a BHer should act really disrespectful or such around sith given they should know how dangerous Sith can be. But its VERY doubtful that the majority of BHers would actually be trying to keep up on their 'Imperial Protocol'
Hell there is that line the BHer character can state early on
"Just a reminder this is an imperial world, if we tweak the local authorities they can make life really hard."
And you CAN reply with
"I like tweaking with authority."
Hell Bioware themselves even describe them as "Its very much the man with the gun against the world."